Listen to the Larry King show and it becomes clear that three o'clock in the morning is prime time for the imagination. Thoughts cultivated in silence and solitude flower in the darkness. The quiet conjures intimacy; the lack of distraction throws a soul back on the shores of itself.

It is good for comtemplation, and for craziness, of course, and for the unexplored corners of the mind. For Larry King, it's a living. He provides a conduit for it all, for all the unblinking eyes, and unheard voices.

Every weeknight, form midnight to 5:30 a.m., he sits on the 12th floor of one of the concrete fortresses of Crystal City, in a small carperted room with a view of the Capitol, an angled table and a raft of telephones and microphones. Only a glass wall separates him from the control room, and only a phone call from the nearly 10 million who listen to him nightly on 139 radio stations across the country.

From midnight until three, there is a guest to question, and after that he throws the microphone open to whatever subject those who call want to discuss. He gets there a few hours ahead of time to look over magazines and newspapers and to start his mind on the restless ricochet that will produce the barrage of questions he asks his guest.

King is proud of his audience, the listeners and callers who phone in at their own expense without the invitation of a toll - free number. "I don't treat this show as if it were taking place at four in the morning. The people out there listening are intelligent and interested in what's going on. They're college students, or they're working night shift somewhere, they're a doctor or a nurse or a security guard, they're on their way to work or just getting off. It's the same variety you'd get in the daytime, only thery're paying more attention. There's not a whole lot else going on, distracting their attention."

King figures he's done over 13,000 interviews in the 22 years he's been on the air. He is 44 and talks in staccato sentences, in between a near constant stream of smoke from the low - tar cigarettes he smokes. He has the features of an ex - fighter on a thin, nervous body; his face is creased with irony and experience.

One of the earliest memories is listening to the radio show "The Shadow," and, as might be suspected of a radio personality, he is something of one himself - elusive in the high noon of philosophical pronouncements, more at ease in the twilight of the vagaries of life, its anecdotes and ironies.

"I'm a survivor," he says. The self - revelations do not come forth like the vignettes and comic routines he likes to render, but values are partially revealed in the men he admires, a disparate lot bound together by a common thread - "The ones who don't cop out. You ask a question and you get an answer."

King's guest one night last week was Rep. Paul McCloskey (R - Calif.), who ambled in blinking sleep from his eyes, the night after catching the red - eye flight back from his district. McCloskey wore a ragged blue pullover and a pair of corduroys - the absence of TV's lights and cameras erased the need for the perfect tie and uncreased smile.

"Coffee," said McCloskey in the voice of a man looking for a life raft.

The Larry King show in its present incarnation began in Washington a year and a half ago, but King started his career in Miami. He went there from Brooklyn, on the advice that it was a good place to break into the radio business.

He began as a deejay on May 1, 1957. He remembers the date. The deejay job led to an on - location show at a local restaurant trying to drum up some morning crowds. "The second day, Don Rickles comes in. So I interviewed him. And everyone else who showed up." He did Bobby Darin and Lenny Bruce and Ella Fitzgerald and a clothing manufacturer and a plumber. "Whoever walked in the door. Ever try to talk somebody about creative plumbing?"

It was time to begin and King began his questions of McCloskey: Does a Northern Virginia congressman get the same travel allowance you do? How did you get involved in politics? Why are you a Repyblican? What was it like to run against Shirley Temple?

An hour glides by. There had been talk of Watergate and Vietnam, McCloskey's idea on the draft, on politics, on his party, and his anecdotes and observations became all the more vivid when painted against the oddness of the hour.

There was a station break and McCloskey paused for breath. "A few more questions like he asks and I'll be wid awake," he said. "I never know what he's going to ask next."

King takes pride in his interviewing. He never reads the books the authors he interviews have written ("I want to be in the same boat as my audience") and doesn't propare questions, depending instead on his own curiosity to reap the answers his unseen audience will find interesting.

Architects are the most outspoken professionals to interview, politicians the least, criminal defense lawyers the most interest - "they hold a person's future in their hands." King has talked to Richard Nixon and Jackie Gleason and Edward Teller and Jimmy Hoffa - the very list is its own peculiar portrait of America.

King interviewed McCloskey for an hour and then announced that he and his guest would now take telephone calls. In less than a minute, all 10 lines were flashing. They would stay that way all night long, pulsing like a heartbeat, the calls coursing from all across the country.

They wanted to know about Robert Kennedy's assassination and Somoza and SALT, about Rhodesia and Keynesian economics and whether McCloskey had a signature machine. There was a censor button on King's phone, but he didn''t use it that night. The callers were not the strange night crawlers late night shows often seem to attract, and King is no Miss Lonely-hearts.

As each call was being taken, someone in the control room held up a card with the name of the city the next call was coming from - Chicago, Danville, Pittsburgh, Pensacola, Buffalo - and King orchestrated them all, cutting it short when necessary, keeping it fast and smooth and interesting.

Some of the callers described themselves - and ex-Teamster, a disabled WAC from World War II, a young balck American, an elderly conservative Republican woman who just wanted King and McCloskey to know that not everyone in her slice of the demographic pie was against the ERA. But, mostly they were just voices, urgent, interested and in need of this blind forum to share thoughts, to let people know.

McCloskey fielded the questions until 3 o'clock, and left describing King as one of the best interviewers he had ever encountered. In turn, King, who describes himself as a Humphrey liberal, looked in McCloskey's direction and said during a commercial, "This is what the country wants. More people like that man, people who tell the truth, who tell them what they really think."

That category holds an incongruous crew, Adlai Stevenson and Frank Sinatra, Barry Goldwater, Muhammad Ali, "when he wasn't allowed to fight," Jimmy Hoffa and Hubert Humphrey "when he wasn't vice president," Andrew Young. But the icon is Lenny Bruce. "I spent a lot of time with him in Miami. He liked Miami, it was the only city he didn't get arrested in. Lenny was a seer. I often wonder what he'd have to say about the decade coming up if he were around now."

Bruce, he said, "just would not cop out. I must have seen his show 25, 30 times and he wouldn't compromise. He was always the outsider."

Did King identify with Bruce's outlaw status? "Yes, but I couldn't have done what he did. I didn't have his guts."

The conversation during the commercials was peppered with anecdotes about sports and comedians. "I've always thought that if I wasn't doing what I was doing, I'd want to be an announcer for a professional baseball team. I think that would be a wonderful life - to be doing something so unimportantly important."

But it is the great comedians that seem to fascinate him most. There is a communion with their freedom to tell their version of the truth under the veil of humor and with their motivation as well. "One of the things I love most about my show is the feeling that, for 5 1/2 hours every night, I'm in control of what's going on. A comic gets that too, but for him, it's not only an instant reaction that he gets from his audience, it's instant love. He either has them or he doesn't. You can't fake laughter."

At 3 in the morning, Larry King opens his show to whoever's listening, and with his permission, they call to talk about whatever's on their minds. On that night the angle was the gas shortage, but in the day's dark womb, the geometry of conversation changes lines break and shape themselves into tangents and triangle and odd little circles.

It makes you realize how small the country is," King said. "We're all drifters now." But late at night, Orlando has a chance to talk to Brooklyn and King gleans a certain sense of what the country is thinking.

He watched the gas shortage creep across the country and he hears about their dissatisfaction an he sees the issues glow bright and warm and fade away. "Things like living together and smoking marijuana just aren't bothering people anymore," he said. "Even issues like abortion aren't mentioned like they used to be. Abortion doesn't mean much if you can't get gas for your car."

That night they talked about the energy crisis, their disbelief in the corporate and governmental explantations for it, their frustation in the face of that disbelief, their sense of helplessness. They offered advice, complaints, and reports. No gas stations open in Cleveland that night, worry in Mobile about "the way they discriminate against us all-nighters, updates from the lonely ones.

"Gee, Larry," said Alexandria, "It seems like 1,001 things happen in between the times I call you, there just isn't enough time to talk about it all." . . . "I call my new long distance radio the Larry King radio," confides Los Angeles, "because you're the only program I listen to. I call you all the time." King is uncomfortable about those who find him a personal friend and confidante, the ones who seem to have no one else to turn to. "I try not to think about that. There are shows that pander to that sort of thing, but it's cheap radio. I don't like it."

By 4 in the morning, King was getting punchy, playing with his listeners, laughing with them. Niland, Calif., called to tell King about his new invention, "a bi-wheel vaporizer - it will give you eight to 30 per cent more gas mileage." Acttually, said King, "I favor a tri-wheel vaporizer myself, I get 37 miles to the gallon going sideways in my Buick, of course I hit a lot of buildings that way, but what are you going to do?"

"No, seriously, Larry," said the inventor, "I've been called a liar by experts."

"So have I," said King.

The call ended, the commercial began and King smiled. "Can you imagine what it would have been likke in 1908 if they had had talk shows? Some guys calls up and says, "My name is Wright and my brother and I just invented this machine that flies." And some guy like me gets on and says, 'Sure, buddy, sure you did.'"

And then to his audience he said, "Well, another hour has slipped away in our never ending search for truth," and there was one more hour yet to go, before Larry King would leave his view of the Capitol for his home in McLean, another hour yet of visions and facts and figures and dreams of conspiracies and flights of fancy, as the night movers called to talk to him and to paint their portraits of the world, sketching their philosophies and etching their opinions and hanging them there on the web of night.

"Oh, Larry," said Grand Rapids sadly. It's all so very confusing, isn't it?"

"Yes," said Larry King. "It is." CAPTION: Picture 1, Larry King, by Joe Heiberger The Washington Post; Picture 2, Larry King, by Joe Heiberger