As darkness settles over the crowded apartment buildings of North Arlington, Mr. X cautiously opens the door to his walk-up flat. There is no nameplate on the door or on the mailbox. His telephone number is not listed.

"I take precautions," says Mr. X, a middle-aged Russian who bears a vague resemblance to Secretary of State Cyrus Vance.

An orange rug covers the floor of his sparsely furnished apartment and a picture of his wife, mother and two grown sons -- all in Moscow -- sits on the table beside the sofa.

"I have always been against the system," says Mr. X, who defected from the Soviet Union two years ago for "ideological reasons."

"When I was a student I criticized it, even during the years of terror. Then during Khrushchev's time, I hoped Russia was changing. They released people from camps. The armed forces were reduced 40 percent, then. . . ." His voice trails off.

"The reason I didn't defect earlier was because my children were young," he says, pouring a glass of Rhine wine and laying the table with typical Russian fare -- pickled cabbage and herring, beets and sour cream. "I miss them, sure; I want to get the Soviet authorities to let them go, but when my son applied for an exit visa, they put him in a psychiatric institution for three week's observation."

In Moscow, Mr. X held a position related to arms control. Here he fills his day by working with right-wing groups concerned about Soviet imperialism and advising members of Congress about the pitfalls of SALT.

Disgruntled Soviets like Mr. X can be more effective watchdogs for the SALT treaties than all the space-age electronic equipment orbiting the earth. A top-level defector could bring news to the West Russian noncompliance with the treaties.

"If the Soviets are contemplating cheating; if they think they can gain strategic advantage, they have to think, 'Can I get away with it? Supposing one of my boys doesn't like it and leaves,'" says former CIA director William Colby.

A spy-in-the-sky can count Soviet missiles, but only a spy on the ground could tell the CIA what the Kremlin intends to do with them. He could also say if the Russians secretly equipped their SS-18 and SS-19 missiles with more than 10 warheads -- in violation of the treaties -- as some senators fear. Are they surreptitiously developing a devastating new weapon -- something which takes 12 years to get off the drawing board and onto the launch pad, where it can be spied on by satellites? Electronic gadgetry can discover much about the enemy's capabilities, but it cannot see through steel. It cannot look into men's minds or learn of high-level policy decisions in the Soviet government.

In 1976, a presidential Foreign Intelligence Advisory Board now defunct, warned that the United States was too dependent on electronic surveillance.

"One well-placed human agent in Castro's government could have provided early warnings of the Soviet brigade's presence and described what its true purpose was," says Cord Meyer, former CIA assistant deputy director of plans, and now a columnist.

Recruiting spies, however, is easier said than done, especially in a totalitarian society such as the Soviet Union, with its closed borders and its watchful KGB.

Soviet intelligence has some built-in advantages when it comes to gathering information with people, not satellite, say experts. "It costs us billions of dollars to collect information on the Soviet Union which the Russians can pick up out of Aviation Weekly for nothing," says Colby. In addition to material on the open market, the Russians have had considerable success buying top-secret information from money-hungry U.S. citizens.

Although the Soviets can find agents in the United States, the United States wins hands-down on defectors. They range from artistic defectors such as Mikhail Baryshnikov or Mstislav Rostropovich, who move easily into U.S. society, to important Soviet and East European officials whose whereabouts, intelligence contributions and adaptation to American life remain one of the darkest secrets of the American spy establishment.

However, since 1975, when Congress began to investigate the CIA, some information has been made public about defectors' lives in America.

These "leaks" are causing alarm. "After all the publicity about what happened to Nosenko and Shadrin," says Dr. Ray Cline, former deputy director of the CIA, "we may have trouble encouraging other defectors."

Yuri Nosenko was a watchdog for the KBG at the U.S.-Soviet Disarmament Conference in Geneva when he defected nearly 16 years ago, just three months after the Kennedy assassination. His assertion then that Lee Harvey Oswald was never in the pay of the Soviets remains controversial today. The information smelled to certain CIA factions of "disinformation," part of a mission to distance the Soviet Union from the assassination. Last year's testimony before the House Assassination Committee revealed that in order to "break" him, the CIA subjected Nosenko to imprisonment for four years, including a period of confinement in a specially constructed 10'x10' windowless vault of steel and concrete. There, to keep his sanity, he fashioned a chess set from the threads of his clothes and tried to keep track of time in the dust. Finally, in 1967, a decision was made to clear him.

Nicholas Shadrin was a high-living Soviet naval commander, who, in 1958, stole his ship's long boat and escaped with his Polish finance, Ewa, in a 24-hour Baltic crossing to Sweden. In the summer of 1966, while working for U.S. naval intelligence in Washington, he was approached by KGB agents to spy for the Soviet Union. The FBI encouraged him to play along, feeding the Russians carefully selected "soft" information. But three years ago, while in Vienna, Austria, to meet a KGB contact, Shadrin mysteriously disappeared.

"The Swedes warned us not to come to the U.S. They use you and dump you," says Ewa Shadrin, her eyes filling with tears as she sits in the living room of their Arlington house surrounded by mementos of their marriage.

Mrs. Shadrin, who believes her husband could still be alive in the Soviet Union, accuses the CIA and the FBI of using him as bait and of botching his surveillance in Vienna.

Most intelligence experts agree with Ray Cline that if disaffected Russians and East Europenas are frightened of becoming espionage casualties like Nosenko and Shadrin, then a vital intelligence source is endangered. On the other hand, a few case officers who have experience with defectors wonder if the information they supply justifies the troubles they bring both to their personal lives and to the agency resettling them in the United States. "You never know if a defector is for real," claims one former intelligence officer. "The Russians have flooded the market with phonies."

The U.S. intelligence's first task is to penetrate the lies, to establish the defector's "bona fides." Is he genuine, or is he a "plant"? His name is run through the computers and an urgent meeting is convened of the Interagency Defectors Board, made up of representatives of the Cia, Defense Intelligence, the military services, the State Department and the FBI. Speed is essential. Once the Russians learn someone is missing, they start agitating with the local authorities. If that country is friendly to Moscow, it may mean smuggling the defector "out black" -- hiding him in the trunk of a diplomatic car or flying in a plane to pick him up.

In the United States high-ranking defectors tend to settle in the Washington area to be near the CIA. "Wringing out" -- debriefing -- can take two years, after which a defector may continue as a "consultant" with a stipend.

"The house was always full of people," recalls Ewa Shadrin. "The guards mostly sat in front of the television smoking. A couple came in to do cooking and cleaning."

Indeed, the CIA and the FBI become the defector's surrogate family, giving new identities, providing jobs and houses, fixing up a divorce and in the case of Arkady Shevchenko, the Soviet's No. 2 man at the U.N., who defected last year, they may have underwritten a call girl.

"The business of hand-holding defectors," says one former CIA officer, "is an obligation imposed for life. It does no good to say 'but I left the agency last year' when they call in the middle of the night. They cling to someone who understands their problems and could do something."

Some defectors come with grandiose ideas of their own importance, expecting Washington to create miracles. One particularly troublesome Russian insisted on becoming a professor, though he didn't want to learn English. "In the end the CIA gave him a $35,000 stipend and found him a special tutor. Then he wanted us to send his kids to private school, then to private college. It was a terrible drag on the agency," says one former CIA agent. c

Another who expected special treatment was Anatoly Golitsyn, who defected from the Soviet embassy in Helsinki in 1960. Allegedly the highest KGB defector ever, he was the man who confirmed that Kim Philby, head of the anti-Soviet section of British Counter Intelligence, was the mysterious "third man" who tipped off Donald Maclean and Guy Burgess that M16 was about to arrest them as Soviet spies. (Anthony Blunt, Queen Elizabeth II's debonair arts curator, was recently identified as the fourth man who likewise warned Philby.) At first, Golitsyn insisted on being debriefed by the U.S. president. The stocky Russian believed a "mole" (a double agent) existed in the CIA. Give him $10 million, he suggested, and he would agree to become chief of NATO counter intelligence.

Successful adjustment to the American way of life largely depends on a defector's reason for leaving his country in the first place. Motives vary, though a large percentage are middle-aged men with marital or drinking problems. Some are attracted to the consumer society. Other defectors come to revenge their country's political system. This is particularly true of East Europeans who want to get back at the Russians.

"The defector who comes for ideological reasons does best," says Konstantine Boldyrey, a Russian emigre who has helped refugees in the United States. "Their ideology is a crutch. Those who come for material reasons usually break down. The intelligence defectors are pretty pathetic; all they are trained to do is spy."

A secret CIA study in the late 1960s on communist defectors' adjustments to American life concludes that the Soviets have the most difficulty. A sentimental people, they become depressed easily and start drinking as they wrestle with guilt and loneliness in a society where individual initiative, not state planning, is the key to success.

Whatever the motive, whatever the adjustment, one thing defectors share is fear. U.S. intelligence officials have been told that every Soviet embassy has a leather-bound "blue book" containing names of traitors sentenced to death in absentia -- the KGB hit-list. Since Stalin's death, the KGB has curtailed its terror tactics, though the Shadrin kidnapping and the Bulgarians' poison umbrella attack in London last year have caused defectors to worry. Today, in an era of partial detente, a tacit agreement has emerged that says, "We won't trouble you if you shut up and don't engage in anti-Soviet activities." Nonetheless, the KGB continues to track down some defectors, trying to "double" them, or pressing them to come home by mailing them copies of Golos Rodiny (Voice of the Motherland).

After a bad start -- his wife's suicide in Russia, scandals with a call girl and a bout with the bottle -- Arkady Shevchenko is determined to prove that a defector can make it in America without a change of identity. Once he completes his memoirs, for which he received a $600,000 advance, he plans to come in out of the cold as a public personality, to lecture, teach, write and speak out on issues. "He knows the risk, but he prefers to live in freedom for as long as he can. That's one reason why he left," says Bill Geimer, the Washington lawyer Shevchenko hired to protect his interest and to quash his playboy image.

Now married to an American, Shevchenko leads a quiet life in a Washington suburb. He has had no contact with the Soviets since three days after his defection, when Anatoly Dobrynin, the Soviet ambassador to Washington, and Oleg Troyanovsky, the Soviet ambassador to the U.N., tried to persuade him to return home.

U.S. intelligence experts say the lives of defectors are often fraught with problems and risks. "But that's the business they chose," Cline says. "I can't feel too sorry for them. They knew the name of the game when they got into it."

Unlike reconnaissance satellites, human spies cannot be turned in for more sophisticated models. As long as the United States and the Soviet Union remain political antagonists, defectors will be the sad but vital pawns in the East-West game of "I Spy." d