Soprano Sylvia McNair joined the National Symphony Orchestra's pops series Saturday night to sing works by such noted American composers as George Gershwin, Duke Ellington and Leonard Bernstein. McNair has made her name singing more traditional classical works and her voice is a fine one indeed, but the selections for this concert brought mixed results.

On Ellington's "Sophisticated Lady," for example, her voice was melodious enough but lacked the snap and sultriness one would expect when the Duke is performed. She seemed more comfortable when the program switched to the showier works by Gershwin, and she gave an absolutely startling rendition of "The Man I Love," a somber and wistful tune from the lesser-known musical "Strike Up the Band."

Although prone to unnecessary flourishes and enunciating lyrics almost too properly at times, McNair handled most of the rest of the program, especially Rodgers and Hammerstein's "Hello, Young Lovers" from "The King and I," with gratifying elegance.

Under the baton of opera conductor Hal France, the orchestra was virtually flawless, whether accompanying McNair or during instrumental interludes. At moments it even took on a spontaneous side, with regrettably brief solos during a medley of Ellington tunes and displaying a whimsical liveliness during three dance episodes from Bernstein's "On the Town." Particularly strong were the horns, which hammered out many a catchy melody on this night.