The National Symphony Orchestra got back to business last night at the Kennedy Center Concert Hall with the official opening program of its 74th season, under the direction of Leonard Slatkin.

Four very different pieces were presented -- a suite from "Der Rosenkavalier" by Richard Strauss; Mendelssohn's Violin Concerto, with soloist Joshua Bell; the American composer Irving Fine's "Notturno"; and Leos Janacek's "Sinfonietta." Indeed, such was the disparity in the quality of the performances that the NSO almost sounded like four separate orchestras.

The Strauss need not long detain us. I'm not sure that trying to cram all the hot tunes from a three-hour opera into the space of 22 minutes was a great idea in the first place. Last night's rendition -- loud, coarse, hurried and undifferentiated -- certainly made a poor case for it.

Matters improved enormously when Bell took the stage for the Mendelssohn. His sweet, plangent tone, abundant virtuosity and agreeably assertive manner were altogether convincing, as was the substantial and appropriate cadenza he composed for the opening movement. Slatkin, who had taken a distracted, laissez-faire approach to the Strauss, was suddenly all concentration, and the performance built from strength to strength. Standing ovations in Washington are pretty much the norm: Bell, Slatkin and the NSO deserved theirs.

To this taste, the most affecting music on the program was Fine's luscious "Notturno" for string orchestra and harp. It is a work in three movements, deeply influenced by neoclassical Stravinsky at his most chaste ("Apollo," for example, or "Orpheus" seem clear predecessors). And yet there is a fresh, distinctly American innocence of utterance to the "Notturno," with its emphasis on the same long, affectless melodic lines we associate with Aaron Copland and Virgil Thomson. Fine died young, at 47, of a heart attack. His music, whenever it is played, is moving and effective. And, as so often in lost American scores, Slatkin proved a sensitive, sympathetic advocate. The playing was delicate throughout, suffused with an elegant melancholy.

The "Sinfonietta" closed the evening -- bright, brilliant, musical cubism that seems to have been constructed block by sonic block. Here, the NSO brass had a chance to shine; the massed trumpets (placed on either side of the stage) would probably have done the job at Jericho.

The concert will be repeated this afternoon at 1:30 and tomorrow night at 8.