HIS AFFINITY with show business began, Joshua Logan suspects, when he was 2 1/2 years old, living in Mansfield, La., with his grandaddy and widowed mother. He doesn't remember the time, but he's been told the story so many times he figures it must be true. One evening he was taken to an evangelist's meeting, and the preacher was ranting on and on about repentance and so forth. Borrowing a phrase that had been directed at him by one of the family help, he shouted in clarion tones and baby innocence: "Gawd, Man, hush!"

For years after he was remembered -- affectionately -- by folks in town as the boy who told Nick Carter to hush up. "Who knew that three words said in public could leave that kind of impression?" he said. "I just knew I was going to go on the stage."

Logan, now approaching 73, went on from that first public display to a life in the theater that approaches legend. He is in Washington for a few weeks, not on a pre-Broadway tryout this time, but for his first attempt at teaching, a three-week workshop in American musical theater at American University's Performing Arts Academy.

The students are in leotards and headbands, their tap or jazz shoes clacking on the floor. Though they are young, they know who Joshua Logan is. They know the songs from "South Pacific," which he directed, co-authored, and co-produced, and the names of the characters in "Picnic," which he directed on Broadway and on the screen, and the plot of "Bus Stop," in which he directed Marilyn Monroe in one of her best roles. Some of them bring copies of his two volumes of autobiography, which are filled with anecdotes about people like Mary Martin, Stanislavsky, Henry Fonda and Rodgers and Hammerstein, to be autographed.

And if they don't remember everything in a career that started in the early 1930s, he tells them. Every day he talks for an hour and a half, reminiscing, hoping to communicate the complexities of the theater profession. Later in the day, he teaches an acting class.

"This is a sort of an experiment," he said recently during a break between classes. "I thought it might give me another book. Some kind of a new way at looking at myself." Also, he added, he is a workaholic. He likes to be busy.

The students seem to regard him with a mixture of affection and respect. They call him "Josh," as he requested, and bring their lunch to class, as he suggested. He sits in a high-leged chair on a stage, half-glasses perched on his head, occasionally looking at his watch to make sure he doesn't talk too long. On the first day, his wife, Nedda, is seated in the last row and shouts out the time whenever he squints at his watch.

A student asks if he can help him down the stairs after one lecture; the offer is ignored as Longa gingerly manages the steps alone. Sometimes he seems to lose himself in mid-sentence; but he always comes back, always remembers the name, the play, the year. He sings songs to them from some of the shows he's done -- the first a rhyming ditty he wrote for Jimmy Stewart in a Princeton University Triangle show. He recalls on of his favorite lines from that show: "I'm just a raspberry seed lodged in the wisdom tooth of time."

When asked what made him so successful as a director, he suggests that perhaps it was his "enthusiasm for something I truly like . . . I can excite everybody else with that and it's also a calming influence on everyone. They trust my enthusiasm. And I trust it. Something else is that I do not despise showmanship. I find it a part of good theater."

The enthusiasm is still apparent. He raves about a young student who did a good reading as Ensign Pulver in "mister Roberts" the other day during class ("That's the only part I would give up absolutely everything to play," Logan says. "He's a totally worthless braggadocio.") But he also has the unusual ability to criticize without being destructive, a quality of openness.

He reveals the showmanship happily while telling about his various directing innovations, like putting a swimming pool on stage for "Wish You Were Here." And the number in "South Pacific" where Mary Martin was washing that man right out of her hair. In the out-of-town performances the number was a flop. Finally, he figured it out: The audience was so entranced with the on-stage shower and soapsuds that the song was ignored. The solution: have her sing the song and then wash her hair. It worked.

"Are you interested in all this talk about Russia?" he asks the class. They assure him they are. "It doesn't have many laughs," he warns -- and launches into a description of his trip there in 1930 to study with the famous director and guru of acting, Stanislavsky. It is one of a series of reminiscences that roll off his tongue like newspapers off a press, each sounding as though he's told them only a few times before.

"Stanislavsky himself was a tall, big man with ver thick lips," Logan says.

"He looked like a foreigner. He was a sort of buff-colored man, with a head of full, white hair -- he was kind, but also a devil. He went after any actor he thought was pulling some silly or worthless thing. He would never let anyone get away with anything that was unrealistic or pretentious."

Logan asked him about "the method" and such devices as "emotional recall" -- in which an actor summons emotional experiences from his own past in order to enrich his performance.

"I don't know what you're talking about," Stanislavsky answered. "Oh no, no, no. That's for the beginning, for the bathroom or the middle of the woods, where you can be alone. You must never use it when acting; it's a means to an end."

From Stanislavsky he learned that he wanted to be a director, Logan says. Before observing the great man at work he had thought of himself primarily as an actor, secondarily as a director. But the trip to Russia, during his senior year at Princeton, showed him the creative possibilities of directing.

Stanislavsky. The Method. That brings him to Lee Strawberg: "I am not at all an admirer of Lee Strawberg. He claimed to be the Stanislavsky man. He said he had never met him and was glad he hadn't . . . There's one thing I hate, and that's pomposity. And he's a pompous ass."

However, Lee Strawberg did Logan "one good turn:" to tell him that Marilyn Monroe was a very good actress. Partly as a result, Logan cast her in "Bus Stop" and believes that she was the "most brilliant"actress he ever worked with. She was almost never late, he says, and always cooperative. He wrote a letter of advice to Laurence Olivier, who was to direct her in "The Prince and the Showgirl," warning him that she was unlike every other actress. "Don't tell her exactly how to read a line," he advised. "Let her work it out some way herself no matter how long it takes." Olivier, he says, did not take his advice, and his experience with Monroe with horrendous.

"For some reason Hollywood doesnht approve of beautiful people like Marilyn Monroe," he tells his student. "She just wasn't the Joan Crawford type of girl. I'll never understand it . . ."

Perhaps, a student suggested, Logan had a special rapport with Monroe because of his own experience with mental illness. By the time he worked with her in the early '50s, he had been hospitalized twice with nervous breakdowns.

"Marilyn was not a manic-depressive," he disagrees. "I am."

He has taken lithium since its use was pioneered in 1969, and feels it is one of the best things that ever happened to him. "One of the luckiest things in my life is that my mental illness, which is something I will have asll my life, has a preventative. I will take it all my life."

He never hid his illness, and went on television with his doctor to help publicize lithium. It was only after his first breakdown that he learned his father, who died after slitting his throat when Logan was 2, may have suffered from the same disease. He says his career did not suffer because of his illness -- probably, he jokes, because show business is a "manic-depressive" profession.

He still see Henry Fonda and Jimmy Stewart, old friends from his college days, but he says they're in failing health. Fonda, Logan thinks, may never act again.

But Logan is chugging along. He did not like his two most recent projects, a play called "Horowitz and Mrs. Washington," which had a brief New York run last year, and a musical called "Look to the Lilies." But he plans to direct a revival of "Charley's Aunt" next year in Georgia and hopes to bring it later to the Kennedy Center. He has an idea for a new musical, too.

This being Washington, he has an appropriate sotry. It has to do with Nancy Reagan, when she was 12 and still Nancy Davis. She had gone to California with her mother and stepfather to visit their friend, actor Walter Huston, whom Logan was trying to persuade to take the lead in Kurt Weill's "Knickerbocker Holiday."

Logan, thinking the girl's boundless enthusiasm might help him snag Huston, asked her to sit in while he played the score for the actor. Huston said he'd do the show if they wrote another song for him; later, Weill produced the immortal "September Song."

And much later still, Logan learned that the young girl he thought was so "wonderful" and enthusiastic had promptly asied, after he walked out the door, "Oh, Uncle Walter, it's a terrible play, it's not for you."

He pauses, chuckling.