Last night the National Symphony Orchestra added yet another impressive achievement to what seems to be rapidly shaping up as the strongest season it has had.

And with this event comes a new twist. Until now the high points of this year's NSO concerts have all been conducted by Mstislav Rostropovich. But the music director is now away until January. And last night's conductor was Hugh Wolff.

The work was that 45-minute complete version of Stravinsky's first behemoth of a ballet, "The Firebird." It is seldom performed this way, because most virtuoso conductors prefer to milk "The Firebird" of its juiciest chestnuts in a suite, thus denying the work its dramatic ebb and flow. Imagine doing this to "Sacre du Printemps."

And if the edition used helped cleanse "Firebird" of the lush mush most commonly heard, so did the quality of the performance. It was clean, fleet Stravinsky, much like the performances of the composer himself and of his friend Ernest Ansermet. Wolff, who is the Symphony's Exxon/Arts Endowment conductor, repeatedly bypassed opportunities to wallow in rich sonorities, in order to maintain the pulse and to keep the lines clear.

The orchestra responded with some of its best playing yet. Stravinsky's enormous range of timbres was never muddled. Balances were impeccable. Some of the trickiest articulations passed without sound of effort, or error. And when the huge sonorities started opening up near the end, there was no need to force the sound.

The Stravinsky followed a playing of the Chopin first piano concerto in which the soloist, Jorge Bolet, in his own elegant, lyrical way, was impeccable stylistically, if not always technically. The orchestra was just the other way around, strong technically, but not stylistically.

The program will be repeated today, tomorrow and Friday.