Wolf Trap's Modern Dance Festival concluded last night at the Meadow Center with a stimulating pairing of two New York-based troupes--Nina Wiener and Dancers, and the Rachel Harms Dance Company--exemplifying a number of trends of the multifarious postmodern era. Both groups are relatively small and new: Wiener works with five dancers, including herself, and established her company in 1976; the Harms ensemble consists of seven, including its founder-director-choreographer, and has been active since 1977. Both troupes are composed of extremely deft, svelte performers so finely tuned to a specific choreographic idiom that it seems like their native tongue, so to speak. Wiener and Harms also exhibit a common concern for movement as pure design, apart from whatever theme or atmosphere the choreography may additionally invoke. Coincidentally, but perhaps not accidentally, both troupes presented works with music by Sergio Cervetti, a gifted Uruguayan composer whose professional career was launched in Washington. He now lives in New York and frequently collaborates with dancers.

Wiener--tall, slender and aristocratically featured--is a charismatic dancer, blending a velvety elegance that suggests haute couture with a slightly derisive chill. Her other dancers are quite individual but they share some of her casual detachment, as does her choreography. "Kemo Sabe," dating from 1979 and virtually a signature work, toys cleverly with the American addiction to horse opera. The movie-Western imagery--pistol shots, lassos, bucking rears and heads--is garnished with a steady racket of "hamboning," the body percussion of snapping fingers, slapped thighs, pounding feet; one whole section of the dance consists of nothing but a seated fugue of such sounds. But the Meadow Center stage proved too cavernous a setting, and the resonant acoustics spoiled the crisp sonics--what should have been a wry satire looked too much like a dry catechism. The more recent, lyrical "Lullabies for Elizabeth," with its Eno-like Cervetti score, projected better. Despite passages of beguiling invention, however, the desultory flow of Tharping slinkings and slouchings eventually seemed aimless and the dancers curiously insulated from each other.

Harms and her troupe convey more of a sense of emotional commitment, though exactly to what remains fairly moot in the context of her fundamentally abstract compositions. The three pieces shown last night were diverse in character, notably original and sharp in conception, distinctly personal in style, and all, to one extent or another, enigmatic. "Rhombos," a trio to a robot rock score by Brooks Williams, features vaguely Oriental costuming and a repertoire of quirky, tic-like movements that gave the dancers the aspect of jiggling blobs on a video screen. "Toxic," a duet for Harms and David Lukcso to spare, acrid music by Eric Valinsky, explores a poisoned relationship--the performers, in gangrenous looking tights, stomp and batter one another but retain an aura of stolid indifference. "Breaking Ground," the most compelling work of the evening, set to a chiming, flutey, drifting score by Cervetti, has the dancers in baggy whites enacting a strangely brooding, obscure parable (about striving and cooperation?) against and upon Miriam Ellner's striking set construction. A slablike platform supported on a ridged, tesselated base, the whole looking like relics of bone, it serves at once as a cave, a series of ladders, and a summit. The dreamy, undulant choreography swerves in, around and atop this mysterious structure, which seems to magnetize the movement and define both its center and periphery. A baffling piece, but an undeniably fascinating one.