Peter Maag sat down during the first number he conducted last night with the National Symphony Orchestra, and he leaned back and watched approvingly while the orchestra played much of the second number with minimal guidance. One might think that he was taking it easy in honor of his birthday--he was 64 yesterday. But no--after intermission, it became clear that he was saving his energy for the First Piano Concerto of Brahms. With a vigorous and subtle solo by Emanuel Ax this work received as high-voltage a performance as ever has loosened the plaster in the Kennedy Center Concert Hall.

Maag was festively decked out in square tails with silk binding on every available edge and a string tie of yards of hanging ribbon. He sat at a harpsichord to conduct a very reduced orchestra in Haydn's graceful Notturno in C to open the program. The harpsichord, though virtually inaudible, added a certain visual e'clat to this charming, lightweight work. Completely audible and delightful were Toshiko Kohno and Rudolph Vrbsky in the Andante, which sounded like a concerto movement for flute and oboe.

In Mozart's great 39th Symphony, Maag took a rather permissive attitude (probably based on thorough rehearsal), which the orchestra, on the whole, justified. The sound was rich and well-balanced from the opening notes of the solemn, slow introduction to the first movement. Ensemble playing was slightly fuzzy at the beginning, but rapidly came into focus. The slow movement evoked some excellent de'tache' phrasing from the violins and some moments of high drama, though it seemed to lose a bit of momentum for a moment. The scherzo was brisk and vigorous, the finale well paced and structured, with some lively work in the woodwinds.

But the climax of the evening was the Brahms, which began like a thunderclap and built to a bold, dramatic climax before Ax made his first soft, pensive entrance. This beginning staked out effectively the dynamic and emotional range of a masterpiece receiving a masterful performance--soft and intimate at some moments; larger than life, impulsive and tense at others.

Ax ranged from gentle whispers to heroic declamation, building his climaxes as carefully at the keyboard as Maag did on the podium. The air fairly crackled with excitement--as much in the understated passages and softly appealing melodies as in the stormy statements. The interpretation went for maximum contrast, and it was remarkable how well it held together, how logically its moods were made to grow out of one another. In an anniversary season that is having a surfeit of Brahms, this performance managed to make the concerto seem fresh and new.