Chopin's Second Piano Concerto is a graceful, flowing treasure of a work and it was in that manner that Eugene Istomin played it last night with the National Symphony Orchestra.

This superb performance marked a bit of a milestone, because Istomin also played the Chopin Second in his National Symphony debut 38 years ago.

No doubt Istomin's command of the notes was just as assured in 1945 as it was last night; but it would have been amazing if his command of the style was so complete back then.

There was the tonal poise of the concerto's treble melodic lines, the lyric elasticity and elegance of the phrasing, and the clarity of articulation. Nothing was forced; runs were wonderfully even. Istomin seemed to be playing down certain critical octaves in order not to disturb the delicacy of scale.

He took the first movement a little slower than it is usually played, but that approach more than paid off because he could bring extra attention to inner voices and harmonic nuances. The poetic slow movement, in particular, was exquisite.

And, for once, the accompaniment was played and conducted (by David Zinman) with style and care. Chopin's orchestral parts are not his strong points and they usually get a blunderbuss treatment.

Zinman, the widely regarded conductor of the Rochester Philharmonic, was making his NSO debut. The orchestra played sensitively for him in the ethereal incidental music that Faure' wrote for Mrs. Patrick Campbell's London production of Maeterlinck's "Pelle'as and Me'lisande." The soft dynamics of the strings at the beginning were quite wonderful. And the wind playing in the famous Sicilienne was a delight--particularly from Toshiko Kohno's flute.

Finally, after intermission, came that behemoth of the hi-fi test industry, Strauss' "Also sprach Zarathustra."

For a work so inordinately popular, it isn't played all that often. One reason is that not very many halls have an adequate organ; that's certainly no trouble at the Kennedy Center. Also not all that many orchestras are up to the tone poem's awesome technical demands. Once again, that was no problem for the National Symphony. Solo passages were superior and the work's torrents of sonority surged forth with ease. Concertmaster William Steck's playing of the many solo violin sections was beautiful. And Steck had to pull a dramatic split-second switch of violins with the player behind him about halfway through, when one of his strings apparently slipped.

The Strauss performance was not the last word in virtuosity--an occasional balance was off, for instance. From this listener's seat the solo trumpet parts were inaudible in the fortissimo sections. Zinman's interpretation was a bit cautious in the first 10 minutes, but then picked up steam.