The first time Henry Fonda tells you he doesn't live the glamorous life, you have your doubts.

Here he is in Washington, playing the role of a Supreme Court Justice in yet another play, "First Monday in October." It is his 53d year in the theater. And, at 72, he is one of those stars whose career continues unbated into old age. He admits that it is almost impossible for him to go anywhere unrecognized.

But after he talks he makes a convincing case that for all of that, his social life is intentionally uncomplicated. Except for his performances, Fonda plans to spend most of this week run in Washington in his hotel suite painting on the easel he has set up near the window; eating the food cooked by his wife, Shirlee; and reading. He is now halfway through "Go East, Young Man," by William O. Douglas, the former justice after who Fonda's role is modeled.

The party invitations roll in. Almost all are turned down because they interfere with his regimen.

Yesterday he didn't plan to go out at all until curtain time. He got up at 10, had breakfast; at noon a film crew arrived to shoot a TV ad promoting the arts; at 1:30 he saw a reporter; at 4 he had lunch, followed by the nap that has been a 25-year ritual; a little after 7 he sets off for the theater, where he can afford to be later than usual because he's wearing no makeup, in an effort to make himself look older. And after the performance ends at 10 he plans to return to the suite for dinner, his main meal of the day. With luck, he says, he will get to sleep about 2 a.m.

He broke his regimen on Monday to attend a lunch at the home of Wolf Trap benefactor Catherine Filene, in the company of Rep. John Brademas (D-Ind.), soprano Anna Moffo, Swedish Ambassador Wilhelm Wachtmeister and several other guests. "But that can't happen very often. It throws me off," he said yesterday.

The weekend, offstage, was entirely confined to the hotel. "I just wanted to paint and watch all that football," he says.

"These constraints are not difficult for me to cope with. They're designed to insure that on a normal work day I'm at may peak between 7:30 and 10 p.m. The only thing that worries me is that it's hard on Shirlee. My reward for this schedule is the exhilaration of those hours on the stage - the greatest sensation that can happen to someone in this profession. Meanwhile Shirlee, a cheerful person who is 30 years his junior and has been his wife for 11 years, "Misses out on the reward and gets almost no social life either."

Fonda is told that but for his hearing aides, which he has worn one in each ear for about a month, he doesn't he looked 10 years ago. "I'm aware of that greatly different from the way that," he says. "I've been very lucky. It's something I inherited from my ancestors - a tall lean figure and an honest face."

"I've never understood that word 'archetype,' he says, and then proceeds to define it. "I never cared to be stereotyped, but it's value that over the years, directors have tended to the archetypical good American - whether it's "Mr. Roberts" or "12 Angry Men." They think of me as representing goodness and the law. I've even played the President twice."

He points even to a Sergio leone vehicle, the spaghetti Western "Once Upon a Time in the West," in which he wasu used as a surprise villain "in a kind of O. Henry effect that's meant to shock people and make them say, 'My God, that's Henry Fonda.'"

Fonda grants that his enduring appeal is in part physical but he aruges that there is a dinstinction between his personal and "the romantic sex sympbols like Tyrone Power earlier or Bob Redford today - not that that keeps them from being fine actors. You saw it in 'All the President's men,' where Redford was a romantic but Dustin Hoffman was just a f - good actor. In my case, I don't think that audiences like the acting rather than that some are having dreams about holding my hand."

Fonda also doubts that he could have endured the coterie of "hairdressers, bodyguards, valets and so on" that often surrounds such figures. I'm not sure that if that had been the case I might not have gotten out of the business. You know, I don't even have a secretary. I counldn't live that way."

As he has gotten older he has found the movies offer him few substantial roles, and because he abhors the notion of retirement he has returned to the stage. "Frankly, I always preferred the stage. There's nothing more satisying than taking a character and developing him from beginning to end. In the movies, everything is done in such fragments that this experience is lost. Also, in recent years I've been awfully lucky to come upon scripts like 'Frist Monday in October' and 'Darrow,' which just arrived in the mail one day from a producer I'd never even heard of.

"I think it's my love for theater that keeps me going back and backs,' he says. "But I couldn't write a book about it or tell any secrets on how to do it. Maybe the right answer is one of the Justice's lines in the play, "it ain't how good you are, it's how long you last."

One consequence of professional longevity is that when Fonda receives the American Film Institute's Life Achievement Award in March he will be the first honored with his career still going full steam.

He says he dreads the nationally televised ceremony. "I try not to think about it. It's just embarrassing to go up there and receive an award. I don't think I'll make a speech. I want to have seated at my table all my children, my four grandchildren, my sister from Omaha and all their wives and husbands, and I'm just going to point to them and say 'That's the best thing that's happened to me in 72 years.'"

Another problem facing Fonda is how he and co-star Jane Alexander will respond to increasing pressures to take "First Monday in October" to Broadway next fall. Originally it was meant for Washington alone, but audiences are enthusiastic, and the producers would like a New York run. Neither star could schedule it before fall.

"It's a fine play for Washington," say Fonda, "but I've got doubts that it would do as well in New York. Certain lines just wouldn't register."

Playwrights Robert E. Lee and Jerome Lawrence have gone off to make some changes in an effort to overcome these worries. Fonda says, "I think we're going to have to wait for the end of the run and see how we feel."

For the spring, at least, Fonda plans to return to their West Coast home (there's another one in New York) and relax. "Shirlee deserves it an d I need it."

The interlude in California, he says will be low key. "i want to get back to my eight dogs, two cats, and several hundred thousand bees," says Fonda, "and also, I never miss springplanting in my organic garden."