Mstislav Rostropovich, knocked out of action by the flu, turned over his baton to young Andrew Litton for this week's concerts with the National Symphony Orchestra. Litton's unplanned debut in a subscription series concert, last night in the Kennedy Center Concert Hall, did not turn into "a star is born," the familiar scenario of show-biz melodrama. His performance was good but not really surprising. And after six months with the NSO as its Exxon/Arts Endowment conductor, appearing frequently in family and pops concerts, the 24-year-old Juilliard graduate is hardly a complete unknown.

The program Rostropovich handed over to Litton was a challenging one: a neglected trifle from Debussy's early years, "Le Triomphe de Bacchus," which was receiving its first professional performance in this country; Edward MacDowell's hyper-romantic and very virtuoso Second Piano Concerto with Andre' Watts as soloist, and Schubert's Ninth Symphony--a work that presents serious challenges even to the greatest and most experienced conductors.

Litton handled his assignment, accepted on short notice, with a fine professionalism, maintaining good tempos and balances, putting the accents in the right places, carefully observing the proper dynamic nuances and building his climaxes carefully and with good effect. Was he nervous? We may assume that he was very nervous, but he did not let it show in his stage bearing or in the music.

In the MacDowell Concerto, the orchestra sometimes (not often) sounded a bit tentative compared with the brilliant, self-assured and highly dynamic performance of Watts. But most orchestras and most conductors would have trouble matching that performance at every point.

From his dramatic opening chords, hammered out tempestuously but with fine precision, Watts dominated the performance. He was tense, exciting, lyrical; his tone whispered and thundered. Notes showered from his keyboard like spring raindrops or volleyed like cannon shots, and the whole splendid display of technique was tempered by a finely honed sense of form and a total involvement in the music's emotional statements. It was remarkable that the orchestra under Litton matched this performance most of the time, and they rightly shared in Watts' standing ovation at the end.

In the Schubert, Litton began well but not exceptionally. The performance was correct rather than distinctive or exciting through the first-movement exposition, but it grew stronger as the music picked up momentum. At first, the dynamic contrasts seemed to be imposed from outside the music rather than to grow naturally from within, but the pace and balances were right, the minor climaxes worked effectively and the movement ended with a real show of inner strength. The second movement began at a high energy level with a good sense of instrumental color, and from there through the headlong finale the performance was highly satisfying.

Litton is still at the beginning of his relationship with this music, and as familiarity grows he can be expected to introduce many refinements, to find ways of expressing his own individuality without violating Schubert's intentions. His interpretation is exciting--any good performance of this music has that quality in abundance--but it will be more exciting in years to come; probably more exciting by the last performance of this program on Friday. Last night was an auspicious debut for a very promising young talent.