Mstislav Rostropovich returned to the National Symphony last night for the final three weeks of the season. He and the players excelled in the works of two composers whose music has not always come easily on the podium to Rostropovich -- Mozart and Brahms.

In Mozart's Piano Concerto No. 26, K. 537 ("Coronation"), about three-quarters of the key to success was the soloist, Alicia de Larrocha. When de Larrocha is really and truly on in her Mozart, no other pianist can exceed the deftness and grace with which she dispatches those pearly runs, those matchless ornaments, those perfectly balanced chords.

No pianist has greater poise, a result of the mixing of the phrasing instincts of a classicist with the variety of color of a romantic. To realize this, just listen to the clear but never overstated way she colors the two main themes of the larghetto. Of the pianistic giants, what other pianist exerts less raw force on the keyboard, with those wrists so perfectly placed above the keys?

Rostropovich's conducting of the orchestra in the "Coronation" was impeccable, with care for balances, not only with the piano but also with the key woodwind details.

His version of the Brahms Third Symphony, that epic of turbulence and reconciliation, was more than impeccable. It was often quite eloquent. The interpretation was broad, a little more so in the second subject of the opening than this listener would have preferred. The deliberate slow movement was just right. And the mysterious lightness that he brought to the scherzo was quite inspired.