You could hold a barn dance in one of the barns at Wolf Trap, and sooner or later they probably will for a fund raiser. The seats downstairs are removable, and the plank floors are ideally made for foot-stomping. Now that it has been tried out in a variety of performances (a bit of Gilbert and Sullivan, baroque and modern chamber music, a film and a piano recital), the Washington music community is getting excited about the potentials of this new performing space. "I don't know how they got those great acoustics," said one local pianist after hearing Earl Wild playing a nine-foot Baldwin there last Saturday. "Shouldn't it sound like a barn?"

Strictly speaking, the new hall is not "The Barn" but "The Barns"; there are two of them, one for performances and one with dressing rooms and a community room, including a bar. Their official name (which the public will undoubtedly not use) is "The Barns of Wolf Trap Foundation," establishing subtly the point that this facility is not a part of the park and the proprietor is not James Watt but Kay Shouse.

The first public attraction there will be almost the only kind of show not already tested: a circus -- a small one, appropriate for a place with less than 400 seats. "It's called the 'No Elephants Circus,' " says a foundation official, and it will give three free performances right after Christmas as "Wolf Trap's Christmas present to everybody." The box office opens for this show on Dec. 18 and 19, noon to 5 with a limit of six tickets per request. For information, call 938-3810, extension 222.