At this year's Oscar telecast, host Johnny Carson got a big laugh by saying, "Someday, we're going to discover just what it is that Jack Valenti does for a living."

"Good question," says Valenti, shorter than average but larger than life in his sunny, book-lined office at the corner of 16th and I streets. "Most people don't understand.

"Johnny is my dear friend, and after the Oscars we had dinner together at Irving Lazar's place at his big party. Carson came up to me and said, "Look, don't think I was too hard on you. I don't know a lot of these people that well, and it's very difficult for me to be funny about them. You're my good friend, so I pick on you. I hope you don't mind.'"

He didn't. In fact, Valenti says, the Oscars did for him what American Express did for Luciano Pavarotti: "After I appeared on the Academy Awards, I find that when I call people in government, I'm better recognized." This despite the fact that the 59-year-old former ad man and bomber pilot has been president of the Motion Picture Association of America for 15 years and was a top aide in the Johnson White House (where in 1965 he became a temporary figure of notoriety for saying, "I sleep each night a little better, a little more confidently because Lyndon Johnson is my president"). But Valenti isn't surprised. "People in government like the dazzle of the movie business," he says. "Politicians and actors are both performers."

He should know. Valenti is one of the best one-man shows in town. He looks like a compact-model Kirk Douglas and talks with a torrential grandiloquence, a veritable wind tunnel of words with a firm opinion on almost any subject:

On the then-threatened directors' strike: "It could have been hard on 'Annie' -- the leading lady might have grown four inches before they started filming."

On attempts to settle the ongoing Writers Guild strike: "Nobody ever gets what they ask for. In a good labor negotiation, both sides feel the outcome is fair."

On marketing movies abroad: "The ingenuity of taxing authorities in foreign countries has no limits. It may be the most creative wellspring in the world."

On the rating system: "It has become the patsy of the movie industry -- it's the most visible part of it, and the most controversial."

On sex and violence on the screen: "Too many parents want to use the movies as a goddam baby sitter. If parents abandon their responsibility, then no Moral Majority, agency of government or rating board is going to salvage that child's conduct."

In a room full of celebrity photos (surmounted by a large drawing of Valenti with LBJ), he will tell you with a straight face and earnest Texas intonation that he sends copies of each of his speeches to a mailing list of the "350 top creative people in Hollywood." (Walter Matthau, he says, wrote to say "Keep sending me those things.") But in the next sentence his voice will grow husky with real gratitude for those who helped an Italian-Catholic kid from Houston make it to the top. He is a man who became "a tennis nut" after starting at the age of 45, who studied Italian while LBJ was napping, who is planning to learn French over his lunch hours, who is still sorry he doesn't have the Latin and Greek background of his WASP classmates at Harvard Business School.

In the gray-worsted gestalt of Washington, he wears clothes that are obstinately courageous for a small man, the sartorial equivalent of a pep rally: a dark, wing-lapel pin-striped suit (with bright blue paisley lining), yellow monogrammed shirt with white collar, fat-knot bronze tie and four-button low-rise vest. On his wrist, a huge, complicated black-face Porsche-design watch with several dials on the dial. "My wife hates it. She says it's bulky and ugly. But I love it. Out of this complexity is a marvelous simplicity."

Wheels within wheels. Valenti is actully three presidents in one, with an annual budget of some $3 million. Aside from the MPAA, he also heads the Association of Motion Picture and Television Producers and the Motion Picture Export Association. The latter occupies most of his life. "i spend about 65 percent of my time on foreign problems. From 45 to 48 percent of all the theatrical revenues of the motion picture industry in America come from markets beyond the rim of our shores." The lush redundancies are pure Valenti: The Demosthenes of I Street is always reaching for the elegant verbal flourish, awash in a deluge of vocables even while jumping up restlessly at intervals to grab a document, suck furiously at his pipe or rattle the change in his pocket like a Metrobus coin-sorter before settling into the elbows-on-knees, forward-leaning posture that in Washington signals the onset of presumptive candor.

"We're in about 115 countries," he says. "The American film literally dominates world screens, for a very simple reason. If you don't like American cars, you can buy German, Swedish or Japanese cars. But there is no known duplicate for the American film."

Not an outrageous opinion for a man who predicts a triumphant return for "the old verities" in American movies, and who says the worst film he ever saw was "Last Year at Marienbad" -- "the critics went into orgasms over it, but it was filled with idiocy" -- and the best "A Man for All Seasons," which after 15 viewings he still finds "an extraordinary document."

On behalf of the 11 big studios (and many television producers) which make up the organization, Valenti girdles the globe negotiating film treaties and related matters directly with foreign governments. "I am really a small State Department," Valenti says. "Every day, cables come from all over the world" containing news that could affect the American motion picture and television export industry, which has a sales volume of $1.2 billion a year and "brings in some $800 million a year in surplus balance of payments" despite home-grown competition.

"Every country is looking for two things: a national airline and a movie industry. It's easier to get an airline. The only way you build a movie industry is through talent -- this sourceless asset that one cannot command to be born. You cannot by edict, bayonet or nuclear threat force somebody to make a good movie."

So pity the poor Third World bureaucrat who invites the mighty wrath of the American movie industry. One example: "We have a serious problem in Thailand, and I've spent a lot of time on it. About three years ago, the Thais had an import duty of two bahts [about 10 cents] per meter of film. At the behest of local film producers, the government raised the duty to 30 bahts. I immediately called a meeting of our companies and decided that we'd send no more films into Thailand until we work this out. We could not allow Thailand to inflict that high a duty on us. If we did, every other Asian country would say, 'Aha -- a new source of revenue!" A celluloid domino theory.

Here at home, "I become," Valenti says, "the voice of the motion picture industry." The MPAA -- with staffs of 60 in New York, 15 in Washington and 20 in Los Angeles -- represents the member studios through its legal and lobbying efforts.

Valenti has faith in this summer's "blockbusters" and in coming releases. ("We've had the gangster era, the horror era, westerns are dead, and the next cycle is comedy -- over the next year and half there are going to be a ton of comedies in the marketplace").

With the box office booming in recent weeks from such hits as "Raiders of the Lost Ark," "Superman II" and "For Your Eyes Only" -- the last weekend in June was the biggest ever for the movies -- Valenti says he thinks the downward trend of the past year and a half has been reversed. "What ails us in the motion picture business has not been an anxiety on the part of the American people, but a lack of pictures that people want to see and I think we've done something about that."

Still, he says, no one knows what will happen from week to week. "We look at the box office grosses with the avidity of witch doctors looking for signs in the entrails of pigs."

This does not mean that he is sanguine about every aspect of the industry. In the Midwest, he is experimenting with the rating system, making available at the box office brief, taped descriptions of the violence, language and sexual content of films. Not in response to any Moral Majority pressure: There has been "absolutely none," he says. When people complain to me that 'they don't make 'em like they used to,' I tell them, 'Fine, don't go to the theater. Pretty soon producers will get the message.'" Exhibiting Dismay

Meanwhile, with increasing competition from home video, exhibitors will have to take account of the "social experience" involved in moviegoing and "clean up their act. You've got to have a theater of 500 seats and a 40-foot screen and garment that enclosure with six-track sound."

Many exhibitors agree, including Marvin Goldman, head of the K-B theater chain here, who says, "I'm building five theaters right now and I know deep in my heart that it's wrong." From 1978 to 1980, Goldman was chairman of the National Association of Theater Owners (NATO), the MPAA's adversary on many issues. NATO, which represents about 8,000 theaters, has sometimes engaged in "almost armed conflict" with the MPAA, says Goldman, who found Valenti to be "a marvelous speaker who thinks very quickly on his feet." Alan Friedberg, the present NATO chairman, calls him "one of the more persuasive and articulate adversaries" and "a consummate politician. I say that with grudging admiration." Chief among the disputes is "blind bidding" (the process whereby theater owners must bid on exhibition rights to a film without having an opportunity to see it) and "splitting," in which exhibitors agree to divide the studios and eliminate competitive bidding.

On behalf of producers, he is fighting a Federal Communications Commission decision which allows those who supply products to cable systems to pick up a program paid for and televised by one station and rebroadcast it around the nation without charge, as Ted Turner's Atlanta superstation does now. Playing Politics

These and a plethora of other problems keep Valenti away from Washington half of every week. "Unless it's an absolute crisis," he tries to get home on weekends to Mary Margaret, 47, his wife of 19 years (the main "creator of the value system for the children," who is now working on a master's degree in clinical psychology at American University) and his three children -- Courtenay, 17, a Madeira alumnus headed for Tufts in the fall; John, 14, a student at St. Albans; and Alexandra, 12, who goes to the Potomac School.

When he is working in town, he prefers not to lobby Congress directly. "I've got very good people who are up there every day and can handle that very well. But when it comes to testifying, I always want to do that myself. vBecause I think I do it better."

He does it so well, in fact, that he has consented to share his secrets with a waiting nation in a how-to book called "Speak Up" (original title: "Tongues of Angels"). "There are literally thousands and thousands of people in public life," Valenti says, voice thick with enthusiasm, "who are abysmal on their feet." LBJ, he says, was "a bore on TV." Nixon was "plastic," Gerald Ford "unable to get his sentences grouped together" and Jimmy Carter had a "sing-song delivery and a way of popping his eyebrows at the end of sentences that was annoying." Reagan, however, is "sensational on the air," although not the forensic equal of FDR: "If he'd been on TV, he would have been elected king."

If the book is sold (with customary beaver-like alacrity, Valenti never waits for a contract to start writing), it will be his third in print, following "The Bitter Taste of Glory" (1971), nine portraits of public figures disappointed in life, and "A Very Human President" (1976), his memoir of the LBJ White House. The Lawyers' Influence

Valenti says his obsession with words began early.Growing up in Houston, "My first great goal in life as a young boy of 12 or 13 was to be a lawyer. I was fascinated by the forensic part of the law, the jousting in the courtroom." ("By a strange coincidence," he says, three people who have been very close to him have been "probably the three most famous lawyers in America. Percy Foreman in my youth, Edward Bennett Williams and Louis Nizer.")

His father was a clerk in the county courthouse, and "I was brought in contact early" with political figures. Valenti would run errands for campaigns, all the while reading about the great barristers of antiquity and discovering, as he pronounces it, Greek "lit-ra-toor." He graduated from high school at 15, a boy "whose ambition -- don't laugh -- since I was 12 years old was to go to Harvard Law School."

But there was no money for that, and he took a job at the Iris Theater, a second-run movie house where he worked 10 or 12 hours a day, seven days a week ("which I never tire of telling my young son, and he gets sick of hearing it") and then became an office boy at the Humble Oil Company, where he soon rose into the advertising department. During World War II he served as a B-25 pilot, and was decorated for his precision bombing over San Nicolo, Italy.

After the war, he returned to the Humble advertising department and finally got his BA in 1946. Among his successes at that time: then "clean restroom" campaign for Humble.

In 1948, with his MBA, he returned to Houston and started his own advertising firm. By 1956, he had caught the eye of Lyndon Johnson. "He was always replacing his palace guard," Valenti says, and each year "made a foray around the state. I was absolutely captivated," says Valenti, who found LBJ "fascinating as a jungle animal." He became "one of the Johnson spear-carriers, and later wrote some speeches for him, and he got to know me well." Valenti put off his dream of running for office for awhile, and "when you put those things off, they never come to pass." He remained on the fringes of politics until the Dallas motorcade of 1963.

"In the basement of that hospital, hysteria was hanging like Spanish moss," Valenti remembers, and one of the entourage said, "The vice president wants you right now." Valenti went to Air Force One, where Johnson "didn't ask me if I wanted to be on his staff. He said, 'As of now, you're working for me.'" Vaulted Onto the Stage

"Life is mostly chance," Valenti says. "The two jolting, cataclysmic experiences which changed my life were totally unplanned. The first was the assassination of the president, which vaulted me onto the largest proscenium stage that I'd ever been on. And second, the day that three movie moguls walked into my office in the White House and asked me to be president of the Motion Picture Association."

The MPAA was looking for a "politically sensitive" and well-connected administrator with foreign affairs experience, and had considered Adlai Stevenson. But Valenti, LBJ's most trusted aide and apologist, was the consensus choice. "They could have gotten an ambassador or an assistant secretary of state or even Dean Rusk if he'd taken the job. But they wanted somebody also who could walk in every chancellery of the world carrying with him the laurel of being the president's friend, total access to the ultimate power. That gives you a cachet which they felt was useful.

"The more I thought about it, the more attractive it became. It wasn't only the money [top White House aides made $30,000 in 1966] although I was heavily in debt at the time. I had also realized that you don't live in the White House forever, it's a transitory occupation." And he no longer had the "fire in my belly" to run for public office.

After 15 years, he is still fascinated by the "creative people" in the movie industry. "I like to sit down with Billy Wilder and talk with him about his script. I enjoy talking about various novels I've read that might or might not make a good movie. I enjoy watching a film before it comes out." In addition to his old friends in Washington -- among them columnist Charles Bartlett, former LBJ protocol chief Lloyd Hand, Rep. Peter Rodino, Sens. Howard Baker and Lloyd Bentsen -- he has made a number of personal friends in the industry, including Charlton Heston, Gregory Peck and especially Kirk Douglas ("He's very literate and speaks five languages -- if you had to spend a week with one person, he would be the guy"). lAnd he is still swelling the roster. "I met Alan Alda for the first time the other night," he says with all the delight of a matinee fan. "He's almost unreal he's so real."

Valenti pauses for a moment in wonder at his own words, but then he's off again, this barrister manque with his unstoppable enthusiasm, gesturing, orating, piling story on story. And all the while, on the face of his big complicated watch, the great hands travel in stately rotation while the little wheels spin and spin and spin.