Antony Dowell, American Ballet Theatre's "lend-lease" star from Great Britain's Royal Ballet, will be dancing for the first time in Mikhail Baryshnikov's production of "The Nutcracker" tonight at Kennedy Center. Before this, the only "Nutcracker" he'd perofrmed in had been Rudolf Nureyev's version for the Royal.

"In Nureyev's production," he says, "the Prince is identified with Drosselmeyer, the magician, and the two roles are taken by the same dancer. In Baryshnikov's stagging, the Prince is also the Nutcracker. So I'll have done all the major roles except Clara-maybe I should go the Trockadero (the all-male travesty troupe) and do her too?"

He flashes that wide, irresistible smile of his, one of the features that, along with his tall, lanky frame, his virtuosity and his stylistic perfection, had made him the Royal's premier classical hero for over a decade. He's joking about the Trockadero, but the quest for adventurous roles and repertory is what led him to join ABT this fall for a year.

"I was very lucky at home," he says. "Everthing in my life had fallen into place. But I was beginning to feel somehow that it was all on one level. I needed the adrenalin of this exciting change."

His first time out with ABT in Washington, last week, he got adventure with a vengeance. Just before his big solo in the finale of another Baryshnikov production-"Don Quixote"-he suffered a severe leg cramp and was unable to return to the stage.

"I must say it was a hell of a shock-not so much the pain of it as the realization that in no way could I continue with the performance.

"You know, it's drummed into you from the cradle-the show must go on. You just don't do things like that." He recouped the "loss," though, in a repeat performance of the smae ballet last Saturday afternoon. "I killed off the dragon, and gave them the full three acts."

"Don Quixote" itself has been another sort of adventure for Dowell, whose image has been tried to that of the lofty aristocraf. Basil, the barber, the role he dances in "Don Quixote," is a skirt-chasing, tipping cut-up.

"It was great fun," he says, "just to have the chance to show my sense of humor, to have a bit of a giggle. It's just as difficulty, in some ways, as the 'serious' parts-ther's a lot of hard dancing in it-but it's real release in comparison to all that cool etuquette and protocol of the classic ballets. And beforehand, I myself had doubted that I could do it. Once you get labeled a certain way it's not easy to break out-I could just hear them saying, 'oh, splendid try, but he should really stick to the classics.'"

He learned the role of Basil not from Baryshnikov-he's never seen the latter in either "Don Quixote" or "Nutcracker" on stage-but from ABT ballet master Jurgen Schneider, with whom he's worked closely ever since his move. "He's become a very important person to me. At the Royal, I'd always worked in an intense way with Michael Somes. I need someone like this, someone I want to please, but who I know won't be pleased easily. And when those really bad days come along, I need that person just to take an interest, to give me that extra push and drive. He's helped me no end, with both 'Nutcracker' and 'Don Quixote.'"

Dowell's rise to prominence was exceptionally swift and smooth. Born and raised in London, he attended the Royal Ballet School and joined the company in 1961, c becoming a principal in 1966. Ashton, Macmillan and Tudor, among others, created major roles for him, and his partnership with ballerina Antoinette Sibley, in such works as "Romeo and Juliet," has the status of legend.

Dowell is 35 now, the same age at which another noted classical dancer, Ivan Nagy, just chose to retire from the stage. "I fight that thought all the time," Dowell says. "The hourglass seems to run out faster the older one gets." But instead he decided to strike out on a new path, in a foreign land, a move he feels was partly caused-as was the change he senses in his dancing of recent years-by family tragedy.

"I'd led this very protected life, and then I had to face up to the fact of both my parents' death. It was all rather horremdous, at first, just coping with things. But afterwards, though I wasn't consciously doing anything, suddenly there it was, all I'd gone through, in my dancing, being used. And I think my performances became much clearer because of this, and perhaps deeper too."