Mstislav Rostropovich and the National Symphony were swinging into "Stars and Stripes Forever," last night on the West Lawn of the Capitol, as the fireworks began about 15 blocks away. The timing was perfect; the pyrotechnic display in the sky came in almost as a visual extension of the music, and the applause for the beloved march merged smoothly into the "oohs" for the lights climbing, dancing and fading above the Washington Monument.

Strictly speaking, the Sousa march was an encore, not part of the program. But it has become part of a tradition, as mandatory in the Fourth of July concert as the fireworks down the street and almost as mandatory as the "Star-Spangled Banner," which was sung by Robert Merrill. Also nearly mandatory was the inclusion of American works on the program: Barber's overture to "The School for Scandal," Gershwin's "Promenade" (with a splendidly swaggering solo by clarinetist Loren Kitt) and the world premiere of a new piece, the three-minute "Fourth of July" march by Andreas Makris, the NSO's composer-in-residence.

Any composer might worry about having a new march premiered on the same program with "Stars and Stripes Forever," but Makris has cleverly composed a march that eludes such comparisons. With an orchestra sitting still on the stage, he uses the music to evoke the sound of a band marching past the audience: first, a piccolo solo, sounding very distant and playing a countermelody to a theme that is never actually heard. As the sound comes "closer," other instruments become audible, until the full orchestra is heard in the middle section, then the sound fades away again, down to the piccolo and then silence. It is a charming novelty and may become another tradition.

The most substantial music on the program was a series of selections from Prokofiev's "Romeo and Juliet," which Rostropovich and the NSO are about to record for Deutsche Grammophon and which should have a solid impact on records.

Merrill also sang Rossini's "Largo al Factotum" splendidly and Gershwin's "I Got Plenty of Nothin' " with fine tone but little dramatic impact. He was best of all, perhaps, in "America the Beautiful," with a substantial part of the enormous audience singing along. Maybe that should become a tradition, too.