James Conlon, the young American just named music director of the Rotterdam Philharmonic, conducted a splendid concert here last night with the National Symphony. It combined two kinds of music sparsely represented in recent years on the orchestra's programs--one is 20th-century French composition and the other the music of Mozart.

First, the French. Ravel's G-major Piano Concerto formed a natural link with the Mozart that came before it. Richard Freed quotes Ravel in the program notes about his intention to write a "virtuoso divertissement, brilliant, clear and light, with sharp contrasts that navigate with Mozartean ease the difficulties . . ."

Last night it was in the most authoritative of hands; the pianist, making her National Symphony debut, was Nicole Henriot-Schweitzer, the prize student of the late Marguerite Long, who played the first performance of the concerto in 1932. Henriot-Schweitzer's interpretation was more romantic and expansive than is often heard. What she lost in momentum, as the madcap rhythms took force, she gained in those "sharp contrasts" that Ravel sought. The slow movement, a cross between a Chopin nocturne and the blues, was very moving.

Conlon ended the program with one of the most undervalued French orchestral blockbusters, Albert Roussel's second suite of material from his 1930 ballet "Bacchus et Ariadne." The virtuosity required to really bring it off is fairly astounding. Last night's performance deserved about a B-plus, with the climax not quite galvanic enough; it's not so much that it should have been louder as that it should have been crept up on a little more coyly.

As for the Mozart, there's no style that's more subtle, or more tricky. Conlon had the right idea. He worked very hard on balances; the all-important winds were never blanketed, either in the "Impresario" Overture or in the "Prague" Symphony. The tone was light, but not as full-bodied as needed. And the overall impression was of tidiness--there was a feeling of Mozart being played without the normal tendency to overexert, with the basic materials in balance, but the music was not yet on that next level of Mozart playing, where tone and harmony seem to pour forth in a cavalcade of the most beautiful tone colors and nuances. It is at that level that people start talking about how Mozart penetrates the mystery of life. The National Symphony and Conlon were not quite there yet.