Each season, the National Symphony slips into a kind of executional lethargy. Everything is all right. But Rostropovich is gone. And too many guest conductors parade before it who are content to settle for less than the best. Then along comes Michael Tilson Thomas. And the NSO takes off like a bat out of hell.

Thomas' two weeks here last season was the high point of the musical year. And last night's opening concert of another two-week stand was just as distinguished.

The National Symphony plays as beautifully for American music's former brash wunderkind now turning master-interpreter as for any conductor. Members of the orchestra greatly respect Thomas; after a magnificently conceived and executed performance of Strauss' "Ein Heldenleben" last season, one often cynical member remarked, "He is just so damn smart." Likewise, when conducting here the immensely gifted Thomas seems to spare nothing of himself.

There was more Strauss last night, "Till Eulenspiegel's Merry Pranks," and it was just as fine. Thomas is a marvelous Strauss conductor. With just four rehearsals he managed to achieve with apparent ease a dual accomplishment: He presented Strauss' incredibly dense maze of instrumental and timbral detail with startling clarity, without in any way sacrificing dramatic impact.

He took some awfully fast tempos in parts of "Till Eulenspiegel," but the orchestra stayed right with him. Part of the reason is the purity of his conducting technique, with a precision of beat that recalls that of Thomas' friend and mentor, Leonard Bernstein.

There were also some unusually slow tempos, but Thomas' control is so complete that he had no difficulties with them, or in making the tricky transitions between them. Thomas did some especially lovely molding of the lyric themes.

Like Strauss the conductor, Thomas is just as good at Mozart as at Strauss. The opening work was the rarely heard Symphony No. 31 in D, K. 297 ("Paris"). Thomas has been specializing in these middle Mozart symphonies (he did another one last season) and he interprets them with incomparable freshness.

The National Symphony has a reputation for being rather ungainly in its Mozart playing. However, it was anything but last night. Those lovely, gallant themes flowed with grace -- especially in the strings. Balances were just right, attacks and releases were exact. It was a joy.

Berg's difficult atonal Three Pieces for Orchestra, Op. 6, featured some fine brass playing, and the elusive, eerie moods were splendidly sustained.

Finally, there was a real rarity, and a tremendous hit with the audience, the Gershwin Second Rhapsody for Orchestra and Piano. Thomas, a fine pianst, played the piano part and conducted as well.

This rhapsody has had a checkered history by comparison with its omnipresent predecessor, "Rhapsody in Blue." And what Thomas produced was something new. As he remarked in an interview last season, "I really know this kind of music." He really does.

Thomas' family was close to the Gershwins, and his father studied piano under the composer.

Gershwin's original parts for the Second Rhapsody were lost, and the existing performing version was one that did not satisfy Thomas. So, with the aid of Ira Gershwin, Thomas attempted to reconstruct the original, and last night's splendid piece was the result. The spectacular cadenza is by Thomas himself. No one brings greater flair to Gershwin's symphonic music than Michael Tilson Thomas -- not even Bernstein -- and it was present in abundance last night.

But then Thomas seems to bring freshness to everything he does these days.

There will be repeats today, tomorrow and Tuesday.