Mstislav Rostropovich, as if to give his National Symphony fans something special with which to remember him until his return next season, opened last night's Kennedy Center concert with a performance of the Fifth Symphony of Tchaikovsky that was little less than stupendous.

With his new, enlarged orchestra more exciting and more of total ensemble with each week, Rostropovich led the familiar music with the utmost freedom. Any time that there might have been a question of the frequent accelerandos and ritards, the score bore out every one the conductor made.

The sound of the orchestra was glorious in every way, from the double basses and violas, with the cellos solidly in the center, through the wind choirs, which were at their finest. When the symphony was over, the audience would not stop applauding for some minutes. At one point, Rostropovich brought horn soloist Edwin Thayer to the front and made him stand on the podium to acknowledge the applause for the beauty of his solo.

With all credit to him, Kitt's clarinet, Kohno's flute and Watkins' oboe were of the same radiance. It was a titantic reading of music some people might have thought was too familiar.

The second half of the evening was no less imposing, since it involved the B Flat Piano Concerto of Brahms with Rudolf Serkin as soloist. Over and over the music came out with melting beauty of tone from both the piano and the orchestra. Soloist and conductor took slower pasages with great expressive freedom, but on occasion the orchestral interludes tended to slow down where a forward movement was needed. And at times there were moments of less than absolute precision.

Serkin tossed off many of the big passages with delicacy and ease. In other places, the taxing power of the music forced him to the limit. In the week's repetitions, with rehearsals behind them, the participants may well find the ideal performance.