Mstislav Rostropovich conducted the Ninth Symphony of Beethoven last night in the Kennedy Center to begin the final week of the National Symphony's 50th anniversary season. It was the first Ninth of the Rostropovich career, but there was no sign of its being new to the conductor. The performances of any great musician are the product of a musical mind at work. To Rostropovich, who has known the great works of Beethoven for most of his life, it is simply a matter of giving tangible form that which has long been germinating -- provided always that he can persuade the hundreds of instrumentalists and singers under his control to reflect his intentions.

Last night the orchestra, and the members of the Choral Arts Society, trained by Norman Scribner to ideal precision and response, were in excellent form. Thus the attention went first to the "how" of Rostropovich's perceptions. His was a clearly thought-out reading, laid out along broad lines, solidly based on what appears in the score. In overall time, Rostropovich took the symphony more slowly than many famous conductors have done, but there was no hint of dilatoriness.

There was extreme attention to details of dynamics and to that flexibility in tempos that is requisite if the music is to live and breathe. Nothing was overstated or neglected. Following a broad first movement, the scherzo was a model of clarity in the contrapuntal voices with solo and chamber passages well illuminated. The slow movement was properly constructed in a single long line that shaped the heart of the work with open affection. Details in it can be enhanced, but it was a loft reading.

There was a miscalculation in the horrendous chord that opens the final movement, both at its first announcement and again upon its return. The timpani overshadowed the orchestra to the degree that Beethoven's shocking harmony could not be heard. This must be adjusted. But the forward motion and the frequent interruptions were superbly handled.

And at last there were the voices. The chorus sang with astounding freedom, letting nothing interfere with their easy projection of the fearful passages. It is rare that four soloists handle the famous assignment with the total musicality and vocal authority displayed last night. Ellen Shade, soprano; Fredda Rakusin, meszzo; John Gilmore, tenor; and James Courtney, baritone, were models.