Curtain call for a modern-day Medici. Roger Stevens, first chairman of both the Kennedy Center and the National Endowment for the Arts, is retiring after 17 years as czar of culture.

WHEN all the applauding hands are still, all the red roses thrown, all the bravos shouted, and all the encores played out, there is still one last bow to take at Kennedy Center. Roger Stevens is bowing out after nearly 17 years as chairman of the white marble performing-arts center on the Potomac. Those who know him don't see Mr. Stevens taking that final bow in top hat, white tie, and tails. He is a shy, unobtrusive man, despite his towering stature and blue searchlight eyes, a man who would rather stand not on stage but at the back of the hall, counting the house.

``The Kennedy Center is the house that Roger built,'' says his successor, Ralph Davidson, the former Time magazine publisher who moves from president to chairman of Kennedy Center in July, when Stevens retires to founder/chairman. ``There wouldn't be any Kennedy Center if it weren't for Roger Stevens.''

Medal of Freedom awarded

President John F. Kennedy must have realized that in 1961, when he made Stevens chairman of a future national cultural center. And President Reagan certainly knew it when he awarded Stevens the Presidential Medal of Freedom earlier this year at a glittering gala. After the tributes, there was a little night music played by Isaac Stern, the violinist and Stevens's longtime friend and co-conspirator in the arts.

Mr. Stern says, ``We were together at the very beginning of the National Endowment for the Arts, which Stevens founded. I was part of the group that chose Roger to be chairman. I would say that he really is the embodiment of what a Medici would have been in our time, a patron of the arts in the very highest sense.''

Roger Lacey Stevens may be a lone skyscraper on the cultural horizon. He also remains a towering enigma to those who know him as not only the man who has made Kennedy Center happen but also a national real estate magnate who once owned the Empire State Building, and a producer (with his partner, Robert Whitehead, or co-producer with others) of more than 250 shows since the 1950s. They include such celebrated plays as ``Tea and Sympathy,'' ``The Visit,'' ``Bus Stop,'' ``A Man for All Seasons,'' ``The Homecoming,'' and the musical ``Annie.'' He considers his biggest successes to be Leonard Bernstein's ``West Side Story'' and ``Ondine,'' by Jean Giradoux, his favorite playwright. Chief among his flops (he says there have been plenty) - he lists ``Colette.''

Stevens has also had a third career, almost as dramatic, in politics and fund raising. He was finance chairman for an Adlai Stevenson presidential run and for the Democratic Party. In the 27 years since JFK tapped him for what later became Kennedy Center, Stevens has raised more than $150 million for it: $80 million to build the center, $50 million to fill its stages, and $25 million for its endowment fund.

But getting Stevens to talk about himself is like getting Garbo to do the Johnny Carson show. In a city of outsize show-biz and political egos, this tall, diffident man is a paradox. He strolls out from behind his desk wearing a brown plaid tweed jacket, gray flannel trousers, and a warm, hesitant smile. Then he leads the way to a round walnut table, where he sits for an interview in which he ducks questions about the interior Stevens but speaks candidly about his triple career and the politics of culture.

As a Stevensonian Democrat, he has sailed blithely through three Republican administrations at the Kennedy Center helm without any political storms. He smiles cryptically when asked about that, then says, ``If someone starts playing political, I just say, `Well, sorry; I'm apolitical.' Stevens, who can look as formidable as the statue in ``Don Giovanni'' when provoked, explains, ``Whenever they would try to get me to talk about politics, I said I just wouldn't get into it, because I didn't see that it had a place in the running of the Kennedy Center. And it seems to have worked.'' Yet he is considered a wizard politician, one who quietly persuaded Congress to expunge $30 million in interest Kennedy Center owed the government by testifying masterfully to the historical inequities that caused the debt.

What about those stories that kept surfacing that Charles Wick, director of the United States Information Agency, was lusting after Stevens's job? ``Well, there was never anything to it. Wick was doing a very good job of handling the overseas arts situation, and he wouldn't have any more wanted this job than the man in the moon. Whenever a newspaper article would appear, I would say, `Charlie, we don't have to manuever on this. Anytime you want this job, it's yours, 'cause I'd like to get out of it.''' Work with noted playwrights

Just across from where Stevens sits is a pile of 50 scripts he's been reading recently. Stevens probably knows more about the care and feeding of playwrights than any other 20th-century American. He's produced the plays of such celebrated writers as Eugene O'Neill, Robert Sherwood, William Inge, Friedrich D"urrenmatt, Jean Giraudoux, Tennessee Williams, T.S. Eliot, No"el Coward, Arthur Miller, Peter Shaffer, Arthur Kopit, and Tom Stoppard. He subsidized Harold Pinter early on, giving him a playwriting grant, and has since produced Pinter's ``The Birthday Party'' and ``The Homecoming.''

If nudged, Stevens will share a story about working with George Bernard Shaw. ``When I first started doing the theater, I was going to put on a play of his called `Getting Married.' ... When we finally got around to seeing Shaw to tie up the rights, in the spring of '50, Shaw rolled over - literally on his deathbed, 'cause he died a month or so later - and said, `You won't open a play of mine in New York in the spring.' And that was it. Those were the days before air conditioning. That was Shaw at his most practical. We had to take it out to the [West] Coast instead. And, of course, in Los Angeles it just didn't work at all.''

``Scripts,'' says Stevens with a snort. ``Everybody says, how do you choose your scripts. I say choose? I say: We finagle. We plan. We do everything we can to get our hands on the good scripts. They don't come to you; you have to go out after them.'' The trouble with `Philadelphia'

And he does, citing the fact that he began grooming Arthur Kopit for a life in the theater when Kopit was still a sophomore at Harvard and later produced his first play. At the Medal of Freedom tribute, Kopit told a story about Stevens's legendary absent-mindedness, describing a stroll down a New York City street together during which Stevens stopped and said, ``Why don't we go into Reuben's restaurant. I've just bought it, and I want to see what it's like.'' When they emerged a couple of hours later, Stevens scanned the Manhattan street and said, ``That's the trouble with Philadelphia - you can never find a taxi when you want one.''

Stevens, a modest man, seemed pleased by the presidential tribute, but guests couldn't be absolutely sure, because he's a notorious mumbler. His wife, Christine Stevens, founder of the Animal Welfare League, says, ``He's pretty stoic, there's no wild uproar or celebration or despair,'' no matter what happens in the life of her husband of 50 years.

Those who have worked closely with him throw some light on the enigma that is Roger Stevens. ``Roger is one of the most positive people I have ever known, always forward-looking, never dwells on past mistakes, errors in judgment or catastrophes,'' says John Gleiber, executive secretary of the Animal Welfare League, who has known the Stevenses for 14 years.

Peter Sellars, former director of the American National Theater at Kennedy Center, says, ``Roger's done more for the arts in this country than any other patron.'' Stevens's partner Robert Whitehead adds, ``Roger is not going gently into this good retirement. He has all sorts of projects and plans.''

For one thing, Stevens is already briskly lining up new productions. He's just bought the rights to William Saroyan's unproduced 1947 play ``Don't Go Away Mad,'' about five veterans in a hospital ward in San Francisco ``who try to help each other.''

Stevens also has an option on a new play titled ``Sullivan and Gilbert,'' about the British team whose operettas are beloved by millions. ``And all we have to do is find two people who are believable as Gilbert and Sullivan. That's like going out in the driveway and prospecting for gold.'' He tosses the line away quietly.

Stevens, a voracious reader, says he's self-educated. Born to an affluent Detroit family, he attended Choate but dropped out of the University of Michigan in his freshman year when the depression wiped out family funds. He worked at a filling station and a Ford assembly line before the Horatio Alger success in depression real estate that made him a multimillionaire.

``Reading has been my avocation,'' says Stevens, who is the new chairman of the National Book Awards. Right now he's zipping through the classic novels of the 19th century, sniffing out potential plays. ``I still think we've missed an awful lot of very good plays that could be gotten out of the classics: Balzac, Dickens, Trollope, the Bront"es.'' Stevens was responsible for the American premi`ere of ``Les Mis'erables'' at Kennedy Center, one of the co-producers of the musical in the US.

There are other gleams in his eye for the future. Stevens, who says glumly that he doesn't enjoy fund raising, despite his talent for it, hopes to raise more money to bring the Kennedy Center Endowment Fund up to $100 million. Also, he would still like to see substantial funding for an American National Theater, which he has backed vigorously over the years, but finds it in stiff financial competition with movies and TV for talent.

A lot of people in Washington think Stevens is going to be a hard act to follow. But Stevens (who doesn't earn even a dollar a year as chairman) differs politely: ``I don't think I'm a hard act to follow at all. I really mean it. I don't think I've run this place anywhere near as efficiently or as well as it could have been run.''

He winds up the interview by mumbling softly, ``Now I gotta go down and raise some money. It never ends.''