It's 3 a.m. one morning this summer and the hotline linking the White House to the Kremlin has a brief, urgent message.

National Security Adviser William Clark is alerted. In three minutes he is dressed, and in another three minutes a black limousine zips him from fe apartment building to the White House, 10 blocks away. He rushes to the bedroom of the president and reads the message from Moscow: "Leonid Brezhnev is dead. Konstantin Chernenko is general secretary."

"What do we know about Chernenko?" asks Ronald Reagan.

Up to this point, the scenario is hypothetical. But the fact is that whoever is going to take the place of Brezhnev--75, ailing for years and almost certain to leave office soon by death or retirement--his biographical sketches are ready: one prepared in the State Department, the other in the CIA. The authors are two Kremlinologists known as "the men with the best shoeboxes in town." They are specialists in the Soviet Communist elite, a glum cabal of paper shufflers and fat generals.

The State Department's man is a cartoonist's image of a federal bureaucrat: frail, bespectacled, with a preference for dark suits, white shirts and inconspicuous ties that never change width. He chooses his words with excruciating prudence, and the minute details of his speciality absorb him totally. He is horrified by the idea that he will be mentioned in the press, and he insists on anonymity. So we'll call him Mr. X.

Mr. X's office is at a dead end of the labyrinth of State Department corridors. Its off- white walls, which could use another coat of paint, are decorated with organizational charts of the Soviet state and party. Towering file cabinets take up two-thirds of his floor space, a forest of every shade from khaki to bilious green that the government has chosen over the past half-century as its color of the moment.

His desk is blanketed with stacks of Soviet newspapers, clippings and files. Next to his telephone is a directory of the Supreme Soviet, their equivalent of the U.S. Congress. Each member has about 50 words of a biography and a low-quality, retouched photograph. The directory is printed in a few thousand copies, Mr. X says, and is extremely difficult to obtain.

On a ledge behind his desk are his famous shoeboxes. They are two gray cardboard boxes, 8 by 10 inches, 5 inches deep, and each box contains some 800 index cards.

"Our system is cheap and fast," Mr. X says, with a shy smile, and he hands over the two boxes to a visitor. The cards are alphabetical. On each card is the career history of a Soviet official, as gathered from public Soviet sources, typed in English or transliterated Russian: schools attended, jobs held, awards received, articles published. Nothing about family, hobbies, interests. That kind of data comes from debriefed defectors and the diplomatic grapevine and is therefore classified, stored in the file cabinets.

The craft Mr. X practices calls for a painstaking culling and inspired linking of pieces of information published over decades. "Some 85 percent of the knowledge any Kremlinologist needs is in the public domain," says a colleague of Mr. X's. "What the profession requires is an encyclopedic memory and lucky hunches." Kremlinologists are rated on their track record of detecting developments--an art that has been called "predicting the past."

For instance, in 1980, the leading contenders to succeed Brezhnev--Chernenko and Andrei Kirilenko--published books, each titled Selected Speeches and Articles. Pravda reviewed the books. But as a careful check by Mr. X's office revealed, only Chernenko's book was reviewed--and praised extravagantly--in the press of the non-Russian socialist republics that together with Russia make up the Union of Soviet Socialist Republics. The conjecture that Mr. X reported was that Kirilenko, No. 2 throughout the 1970s, is slipping, and Chernenko is headed to take his place.

Indeed, one year later, at the party congress, Chernenko sat to Brezhnev's left. (Mikhail Suslov sat to his right, a position that confers a bit more honor, but Suslov was then the senior Politburo member. He died last January, at 80.) Brezhnev and Chernenko chatted frequently--another unmistakable sign of preference. The setup, televised nationwide, told the people that Chernenko is Brezhnev's choice for a successor.

Over the past seven years, as reports of Brezhnev's declining health surfaced in the Western press (though never in the Communist press), Chernenko has become known as the fellow who is always next to Brezhnev when he has problems getting up from a sofa or negotiating steps. Experts currently rate Chernenko as having a 50 percent chance of succeeding Brezhnev, with Yuri Andropov, the KGB boss for the past 15 years and promoted in May as the party's theoretician, as his strongest rival. Viktor Grishin, the Moscow Party boss, is the dark horse candidate.

News of Brezhnev's critical illness hit top-secret cables in the mid-'70s. The first source was a Russian-speaking Egyptian diplomat who heard it from a garrulous Brezhnev in the hospital. Since then, Brezhnev's own complaints to Western leaders--he is a hypochondriac--as well as studies of photographs have suggested that he has a jaw ailment, perhaps cancer, and definitely a heart disease.

Mr. X says that Brezhnev is capable of sustained work for no more than two hours a day. From what Mr. X and his colleagues say, it appears that Chernenko is the keeper of Brezhnev's door, his appointments secretary, his valet.

"Chernenko is Brezhnev's first choice," Mr. X says. "But it is not clear if Brezhnev can dictate the succession." Mr. X believes that if Kirilenko, 75 and three months older than Brezhnev, had not suffered a stroke earlier this year, he would defeat Chernenko, 70, and 16 years his junior in years of service on the Politburo. Mr. X says that the decisive question is whether Kirilenko's camp will switch to Chernenko.

The Politburo is the party's all-powerful directorate, with 13 members at the moment, which selects its boss, the party's general secretary.

What Mr. X's index cards tell is that Brezhnev and Chernenko have worked together for more than 30 years, going back to the days when Brezhnev was party secretary of the Moldavian republic, one of the smaller Soviet republics, and Chernenko was in charge of the party's local "agitation and propaganda" department, known as agitprop, which corresponds to public relations in the West. When Brezhnev moved to Moscow in 1956, he remembered Chernenko and got him the agitprop job on the national level. In 1965, a year after Brezhnev succeeded Nikita Khrushchev, Chernenko was promoted to be chief of the general department of the Party's Central Committee--an administrative organ that issues party cards, keeps records and handles letters. In 1978, after an unusually brief period of 19 months as a "candidate member," Chernenko was elevated to the Politburo.

The most valuable data on Soviet leaders come from Soviet newspapers, Mr. X says, "and Pravda tells you more than any other source. I read it every day." But, he cautions, one has to know how to read Pravda, the official Party paper, which is not written for the ordinary Russian, but "in the language of 'party Pravda,' which has its own clich,es and subtleties." Pravda is "a court publication" observing "a Byzantine protocol" in which "forms become very rigid, and for that reason they become carriers of meaning."

But aren't satellites and spies more valuable than media analyses to size up the next man in the Kremlin with a finger on the nuclear trigger?

"We have to read the press, where he's been, who his friends are--to get a feel for his personality is more important than computer data," says William Hyland, a former aide to Henry Kissinger. "The obvious tools are the real tools."

Hyland is a veteran of the CIA and the National Security Council, now at the Carnegie Endowment for International Peace. What really matters, he says, is whether a Soviet official has been associated with the military (as Brezhnev has been all his life) or sat at his desk making himself indispensable to his boss (as Chernenko has). Power can be measured by a leader's ability to promote people he worked with (both Brezhnev and Chernenko have excelled).

"Chernenko is a consummate bureaucrat," Hyland says. "An inside man for a very long time. He is similar to the personal aide who becomes the chief executive officer. He knows what needs to be done but has no roots and no strength of his own."

The single most significant contact Chernenko has had with an American official took place during a state dinner in June 1979, at the Soviet Embassy in Vienna. The occasion was the signing of SALT II; the dinner was in honor of President Jimmy Carter.

For three hours, Chernenko sat across the table from Malcolm Toon, the Russian-speaking U.S. ambassador in Moscow, who tried hard to strike up a conversation. Nods and grunts from Chernenko established that this was his first visit to Vienna and that the weather was to his liking. "But he just would not discuss anything substantive such as the treaty we signed," Toon says. "He was not unpleasant, only uncomfortable."

Toon remembers not a single smile flickering across Chernenko's expressionless face. Stolid and somber, he ate his way through the 10 courses, from pheasant-in-flight to strawberry plombi,ere, from the consomm,e with Siberian dumplings to the sturgeon ,a la Russe and roast suckling pig.

(At the same dinner, Brezhnev's jaw muscles would not function well and food kept falling out of his mouth.)

During his three tours of duty in Moscow totaling seven years, Toon failed in his efforts to meet with Politburo members--except for Foreign Minister Andrey Gromyko and for Brezhnev, whom he saw a record eight times during his tenure as ambassador, between 1976 and 1979. "I kept asking for appointments with members of the Politburo, but didn't get any," Toon says. "It was one of the things that really bothered me.

"Chernenko is a faceless apparatchik," says Toon. (Apparatchik is the Russian word for bureaucrat, and it is the Kremlinologist's synonym for servility.) "He has consistently played the role of an aide to Brezhnev. He has never put his stamp on anything. If Brezhnev dies, Chernenko may take over in the interim. But in the long run . . ." Toon thrusts his palms upward in a gesture of hard unpredictability.

"If Brezhnev died today and President Reagan would call me to find out what kind of person Chernenko is," Toon says, "I would have to say I don't know."

The failure to get to know Soviet leaders is a permanent American frustration. The conflict is between a closed society's instinct for concealment and an open society's demand for exposure. The Soviets insist on hagiography; the West looks for the lowdown.

Foreigners don't receive invitations to the private residences of Politburo members or meet their wives. There is no sharing of a bottle of choice wine or spending a weekend together in the country to develop the cordiality that is the rule among Western leaders. Soviet protocol calls for their leaders' points of personal contact to be kept to a few words exchanged at a reception.

"It took us years to establish for certain what Brezhnev's birthday was," says Mr. X. "We don't even know Mrs. Chernenko's name."

Communists condemn as "prurient" Western interest in private lives. "The forces of history are objective," a Soviet Embassy press spokesman explains. "It doesn't matter whether someone in charge smokes a cigarette or a pipe." He says that journalistic inquiries aimed at detecting "subjective influences," such as personality traits, amount to "bourgeois sensationalism."

Allied governments share information about Soviet leaders with the United States, but the data are classified for fear of "burning" sources.

One such instance of classified information concerns Mrs. Nikita Khrushchev. When she visited Paris about 20 years ago with her husband, she went on a tour of Paris. As she entered Notre Dame, her French escort officer was astonished to see her making a tiny, hurried, but unmistakable sign of the cross.

The incident was reported to the U.S. government but has been kept secret ever since-- and never published until now --for fear of embarrassing the Khrushchevs, despite Khrushchev fall from power in 1964 and death in 1971.

A similar rationale prevents the U.S. government from releasing any part of the report filed by the State Department escort officer who accompanied Chernenko during his two-day visit in Washington in 1969. (He was in this country to attend a U.N. session and came to Washington to see his daughter, married to a diplomat then at the Soviet Embassy.) To discuss that report would be "inappropriate," says a U.S. official who read it. "If the report said Chernenko is a stupid jerk, it would not be something we would want to make public. But, it so happens, there is nothing fascinating about the report."

According to another U.S. official who read the report, in 1969 Chernenko was interested in a data retrieval system big enough to store information on 20 million people--which confirmed a Kremlinological contention that 20 million is the maximum the Party has set for its membership.

At the U.S. Bureau of Census, one official recalls visitor Chernenko as "just another dull Soviet." At IBM, a spokeswoman says that files such as visitors' logs are shredded after three years.

"We gave him what he wanted," says Mr. X, recalling American impressions of Chernenko as "a dour Soviet type who asked a lot of questions about data banks but gave us nothing in exchange." Which is the usual tradeoff in U.S.- Soviet contacts, Mr. X adds.

Last year Chernenko represented the Soviet Party at the French Communist Party's congress, and the French government passed on its impressions to the U.S. government, as did West German leaders who observed him during their visits to Moscow.

"What we have on any Soviet leader is highly idiosyncratic," says Mr. X. "Riddled with holes. But I know where the holes are. It's an archaic, hand-operated, paper-and-pencil system. But there is no real alternative to it." He explains that his office has no budget for a computer, but even if it did, a computerized system would be six months behind events.

The process is laborious, he says. The Soviet Union publishes hundreds of newspapers that the U.S. Embassy has to collect and mail from Moscow; the material is then distributed, mostly to nongovernment experts who run a flourishing cottage industry in translation and analysis, indexing and cross-indexing. The translations are done, for an average of $30 for 200 words, by thousands of graduate students and retired Foreign Service officers, East European emigres and ordinary people who happen to know Russian. The analysts contracted to research the Soviet press--say, study all references to Politburo members--are scattered among think tanks and universities; they number in the hundreds.

What Mr. X would not discuss what he knows from sources other than press analyses. "I am not going to tell you what else I know about Chernenko," he says, a mite churlish.

"You have to forgive him," says a colleague who admires Mr. X as one of the last of the vanishing breed of Kremlinological craftsmen and is troubled that no one is being trained as a successor to Mr. X. "Anybody who for 20 years reads the Pravda and wades through all that Soviet muck every day has got to be a little crazy."

Across the Potomac at McLean, rules are even more stringent. The CIA turned down a request to interview Mr. X's CIA equivalent, singled out by his colleagues as another old-fashioned craftsman with shoeboxes, which, according to Mr. X, contain far more material than his because the CIA has a larger budget for such operations.

But Washington is not monolithic in sharing Moscow's penchant for secrecy. A little diligence uncovers enough new information about Chernenko to paint a cameo portrait:

The dark blue suits that Brezhnev and Chernenko prefer are cut by the same Moscow tailor from a stock of top-quality English cloth available only to the party elite. The style is conservative, but was never the fashion--anywhere.

Chernenko has gray eyes, high cheekbones and a full head of gray hair. He is the same height as Brezhnev, about 5 foot 8, and just as stocky. Like Brezhnev, Chernenko is poorly educated and is a graduate of a low-quality high school.

Though his name is Ukrainian, he is believed to be an ethnic Russian whose parents migrated to Siberia. His Russian is coarse, uneducated. He is not very articulate in private conversations and is a poor speaker who keeps stumbling over words even in front of a small audience. He lacks Brezhnev's warmth and boisterous self-confidence. Unlike Brezhnev, he is a moderate in his smoking and drinking.

Chernenko is the classic party bureaucrat who spent his life in the cloistered world of the party hierarchy where obedience is the cardinal rule. He has always worked in someone else's shadow and has never had the responsibility to run a factory or an army unit.

His apartment is right underneath Brezhnev's, on a well- guarded section of a Moscow street named after General Suvorov,,the Russian hero who dodged and ultimately defeated Napoleon.

Brezhnev's kitchen is where Chernenko regularly makes his appearance, and matters of state are often decided there. The kitchen is accessible only to the men closest to Brezhnev. Moreover, Brezhnev and Chernenko have been taking their vacations together for years, usually in a Black Sea resort reserved for Communist dignitaries. Their wives are friends.

A Soviet joke, reported by U.S. diplomats, sums up their partnership: Brezhnev is dead. Actually, he has been dead for quite some time, but Chernenko hasn't told him yet.

Thoughtful Kremlinologists believe that Chernenko may turn out to be more of a consensus-builder than Brezhnev. Chernenko has been taking the unorthodox position that the Party must listen to the non- Party masses, and he has been able to translate that abstract principle into practice. For instance, he upgraded the position of the bureaucrat who deals with letters sent to the Party or to Brezhnev and suggested that letters from ordinary people be answered, rather than thrown in the wastebasket. In effect he revived the practice of giving a hearing to the person who in his desperation petitioned the czar.

Chernenko has gone as far as criticizing the practice of each party meeting's having a set agenda and speeches. He has called for candid discussions-- or in one overused yet explosive word: democratization. He seems to be a reformer troubled by class and minority tensions, rather than a hard-liner calling for a crackdown on malcontents.

But all this is Kremlinology, a dowser's search for the precious metal of personal convictions among ash heaps of clich,es. The problem is that Soviet noncomformists and Westerner liberals alike look for any hint of a variation from the dull standard; to detect a divergence is not just a coup for the observer but a sign of hope for the future.

An alternative analysis--in Kremlinology there is always an alternative analysis that is equally convincing--regards Chernenko as a disciplinarian, a hard-line apparatchik. Elizabeth Teague, a researcher for Radio Free Europe-Radio Liberty, argues that Chernenko "proposes firmer discipline as a universal panacea for all the U.S.S.R.'s social and economic worries."

A Soviet leader meticulously edits his text to disguise any private sentiment. There is no hint in speeches Khrushchev made while gletStalin was still alive that Khrushchev was anything but an ardent Stalinist. Similarly, Brezhnev was Khrushchev's most faithful follower until Brezhnev staged his coup.

Soviet leaders are free to express their personalities only if they reach the top. Khrushchev turned out to be insecure, impatient, impulsive. Brezhnev has been an ebullient, skillful promoter of a military-industrial alliance. He has carefully placed in positions of authority men beholden to him.

The West knew moreabout Khrushchev and Brezhnev at the time they took over than it now knows about the people likeliest to succeed Brezhnev, says Paul Smith, editor of the scholarly bimonthly Problems of Communism, published by the U.S. government's International Communications Agency. "We have more information now," Smith says, "because over the past 10 years, Western bankers and senators, foreign policy experts and celebrities of the artistic world have had far more access to the Soviets than in previous decades. Yes, we have more information, but the conclusions are less reliable because many of the skilled professionals who parsed every sentence in Pravda and puzzled over changes in the official biographies have been replaced by the social scientists studying structure and process. The computer printout with quantifiable analyses has taken the place of the in-depth study mixing hard and soft data, speculation and intuition based on files maintained over many years. No wonder we are short on good studies of Soviet personalities."

Toon believes that we could reduce our ignorance by ending "the Soviet practice of having their Washington embassy preside over the bulk of U.S.- Soviet contacts." The U.S. Embassy in Moscow has one of the best qualified staffs of any U.S. mission, Toon says, yet each administration in Washington fails "to provide them with pegs to contact Soviet officials."

There are limits, though, to the values of personal contact. Toon recalls President Carter's first--and only--summit meeting with Brezhnev, in Vienna in 1979. After sitting through the boredom of the the formal dinner the Soviets held for the two delegations, Carter thought he had a better idea. He decided that he would break through the Soviets' reserve and conquer by personal diplomacy.

The dinner tendered by the Americans was to be a small and intimate party; each leader would be accompanied by only three aides. Such a setting ought to encourage good feelings and confidences, Carter thought.

But dinner for eight was not the Soviets' idea of business or pleasure.

Perhaps Brezhnev had been on medication, Toon says, or he might have had too many drinks at the dinner table, but in 15 minutes he was incoherent. The meal was completed within one hour, then the participants fled, both sides conceding defeat.