It was Aaron Copland time again last night at Wolf Trap and there was "standing room only", with very little of that. All the seats and all the green grass for acres around were covered with people come to watch Copland conduct the National Symphony in his own music.

As if to take in almost his entire creative career, the great American musician programmed his piano concerto, which is 50 years old this season, and three Latin-American Sketches which he only put into final form five years ago. There was also his great score," Appalachian Spring," which had its premiere in Washington in 1944, music from "Our Town," and from the ballet "Rodeo."

Copland is an exciting musician to watch as well as hear. During the piano concerto, his genius was recalled in three ways. Not only did he write the fascinating music, but also he does a terrific job of conducting its jazzy complexities, and he could just as easily sit down at the piano and tear off its swinging solos.

For his soloist last night, however, Copland had a young man from Pennsylvania, Gary Steigerwalt, a recent winner of the Liszt-Bartok Competition in Budapest, who took on the blues in the concerto as comfortably as he knocked over its big, 1920's syncopations.

Copeland and the National Symphony have come to know each other very well in recent years. Last night's orchestral playing ranged from just the right luminous quality in "Appalachian Spring" to all the bounce and fun needed for the "Rodeo" dances. Clearly relaxed in spirit, Copeland looked as if he were enjoying himself the whole time. The audience, some of whom stood up at his first entrance, gave him thunderous applause at every chance.