What an evening! It was all Mozart, with the U.S. premiere of music he wrote when he was 9 followed by one ech of his greatest piano concertos and symphonies.

To be truthful about it, the Mostly Mozart Festival Orchestra did not give the U.S. premiere of Mozart's third symphony, now officially listed as K. 19a, last night at the Kennedy Center. The White House preempted that honor by having it played for an invited audience late yesterday afternoon. But last night's sold-out audience heard this country's first public performance of the symphony.

In musical content there is nothing unusual bout the symphony or how it goes its happy way, gracefully following the conventions of 1765 when Mozart wrote it in London. But for it to be the work of a 9-year-old boy is staggering.

Leonard Slatkin performed the function proper in that era of conducting from the harpsichord. It would be interesting to hear the music again with a smaller body of strings, but in every stylistic aspect the performance was ideal.

The evening's greatness came flooding in when Alicia de Larrocha came onto the stage to play the C Minor Piano Concerto. Here is music that Beethoven called unapproachable, and Beethoven was right. It is filled with drama, an element always present when Mozart turns to a minor key. There are exquisite exchanges among soloist, wind choir -- which properly took its own bow at the end -- and strings.

De Larrocha has mastered many things, Mozart among them. There are other ways of playing this concerto, but surely none more beautiful. She shaped and shaded every phrase, giving to each the essential sound and proper place in the whole.Another evening she might find greater turbulence in the music that last night had softer edges than are customary. There are also more interesting cadenzas than those she chose, but which she played in exemplary fashion.

Slatkin was, in both the concerto and the concluding Jupiter Symphony, an ideal Mozart conductor, a breed not overlarge these days. His pacing, his feeling for balances and his shading of cadences were reminiscent of Beecham and Szell who were, though dissimilar, superb Mozarteans.

The orchestra's response to Slatkin was instant and highly informed. Jeffery Khaner's flute shone throughout the evening, as did his colleagues, though there were times when the oboe's tone took on an inconsistency that did not balance with its neighbors, even to the point of threatening the intonation.