Sooner or later, interviewers always ask Kitty Carlisle Hart about the Marx Brothers.

Never mind that she heads the vast New York State Council on the Arts, that she is a walking compendium on the American musical, was married to one of its geniuses, Moss Hart, and knew everybody but everybody, from Gershwin to Sondheim -- and last night gave the third in the Frank Doubleday lecture series at the Smithsonian. It's the Marx Brothers people want to hear about.

She was the one, you may remember, she sang "Alone" with Allan Jones in "A Night at the Opera." She is very nice about it all.

"Don't believe that story about how they made it up as they went along," she said yesterday. "You don't make a classic that way. They went up and down the West Coast with it on stage, tried it in vaudeville. They tested every joke, every gag. Groucho would try out a joke on me, and if I didn't laugh he'd change it and try again, and again."

Even the stateroom scene (where about 40 people crowd into a cabin the size of a trunk and finally burst out like the Johnstown flood) was choregraphed down to the last fat steward, the last huge room-service tray, the last hard-boiled egg. Groucho was the worrier, Hart said. Chico always had a card game or a woman, or both, in his dressing room. Harpo, the great wit of the family, ran with the Broadway crowd, the Algonquin Round Table and other bright people.

"I guess the musical was one of those explosions of creativity," Hart said. "After the '40s it sort of petered out. We don't seem to see the same vitality anymore. The wit. But that's life."

Vitality . . . that would be George Gershwin, the man who said that if he lived a thousand years he still couldn't write down all the songs he had in his head.

"George had a little waltz that he never published, and when he was going with some girl he'd play it and insert her name in it and say, 'I wrote it just for you.' It had a black and yellow cover with some triangles on it. I'm not sayiny he ever played if for me."

There were songs in last night's lecture, too. It is part of a series called "From the Top: Sightlines on Music in America." Hart, who played Prince Orlofsky in "Die Fledermaus" at the Metropolitan Opera and created Benjamin Britten's Lucretia in "The Rape of Lucretia," broke into "Love in Bloom," "Just One of Those Things" and other lovlies and, to show how universal a musical can be, played tapes from "Fiddler on the Roof" in Japanese, French, Dutch and Hebrew.