In his farewell appearance as associate conductor of the National Symphony Orchestra, Andrew Litton undertook a long, emotionally charged program at Wolf Trap Saturday night that placed him at the keyboard almost as much as on the podium. The young artist, who had joined the orchestra as its Exxon/Arts Endowment conductor, will go on to become principal guest conductor of England's Bournemouth Municipal Orchestra.

Devoted to works of George Gershwin and Ravel, the concert called for affinity with the jazz idiom, something Litton has in abundance. His obvious pleasure with the music helped fashion the performance into a festive event.

Ravel's "La Valse," which turns the Viennese dance from carefree pleasure to bitter despair, was suave and flexible, but little orchestral color accompanied the neatly delineated rhythms. In the "Mother Goose" suite, instrumental texture was left apparent. Litton conducted the Piano Concerto in G major from the keyboard with impressive concentration. The second movement's lengthy solo introduction revealed Litton's athletic touch, but he conveyed the presto finale's lightness and quick spirit.

Gershwin dominated the second half, with "Rhapsody in Blue" (in the original version for piano and jazz band), "An American in Paris" and the "Who Cares?" ballet suite of songs from several musicals. "Rhapsody" was crisp and jaunty, with a fine brass sound and a fittingly poetic middle section. "An American in Paris" contained brisk, energetic playing, full of clarity and excellent balance. The final suite, "Who Cares?," needed tighter control and a less boisterous approach, although Litton's piano playing was exceptionally alert and idiomatic.