There were tostados, Krautku'cher and Bordeaux aplenty on the West Lawn of the Capitol last night. But the music supplied by the National Symphony Orchestra was as American as the chicken legs and six packs favored by most in the holiday crowd.

Composer Morton Gould, guest-conducting the orchestra's free Memorial Day concert, chose works as broad and bountiful as the land they were meant to celebrate: Turok's "A Sousa Overture," Gould's "An American Ballad" and Dvorak's majestic "New World" Symphony. It was a rousing, roundhouse punch of a blast to usher in the summer.

It was also enough to bring the crowd of 35,000, which blanketed the grass and crammed the Capitol steps, to its feet--casting aside, for the moment, games of Scrabble, Boggle and backgammon, as well the contents of countless covered dishes and ice chests, bread loaves the size of baseball bats and potato chips packaged in airtight cans (the kind they use for tennis balls.)

"For an outdoor concert, you wouldn't do anything too complicated, or something that has in its texture too many subtleties," Gould said during the break, mopping his brow and puffing one of his beloved Gaulois cigarettes in a trailer behind the bandshell.

The orchestra had surprised Gould, who will be 70 in December, with a slightly early rendition of "Happy Birthday," in which the audience joined. "It was very sweet but I'm not 70 yet," protested Gould, a familiar figure in Washington who, since 1957, has been a frequent guest conductor with the orchestra. "After that song, they should have wheeled me out."

For once the weather behaved. Stubbornly gloomy for most of the weekend, it decided to match the prevailing bubbly mood. "By special intervention of Interior Secretary James Watt, the skies are clear," television broadcaster Jackson Bain, the evening's emcee, suggested. Others offered different theories for the bright-blue sky dappled with cirrus clouds.

"It's all because of the praying of our good pastor here," said Dr. John Karefa-Smart of the African nation of Sierra Leone, among those sitting front and center with the Rev. Lesley Northup of St. Augustine's Episcopal Church.

Northup demurred. "I'm in sales. I'm not in procurement."

Before the music started, Bob French, making the most of the festivities, stood near the bandshell in front of the crowd hawking helium-filled balloons for $1 apiece--until a Capitol police officer smilingly told him he could leave or "go straight to jail."

"If I were rich, I'd just give them away," French, the only vendor in sight, said as he departed for Georgetown. He vowed to return in time to hear the Dvorak.

He would have been well rewarded if he had. The Dvorak, resounding through an amplification system that worked wonderfully for the most part, was the evening's high point.

Without the benefit of outdoor rehearsal--it was rained out--Gould coaxed a nicely balanced tone and a beautifully shaped performance from the orchestra--building, wave-like, to one crest after another. There was well-earned applause after every movement--particularly the unhurried but dynamically building Largo--and a boffo grand finale.

Gould's work and Turok's are in the honorable tradition of composers such as Ives and Copland, plumbing American standards for thematic material, and making fresh music out of the tried and true. The orchestra brought off these works with spirit and aplomb.