The 1985 Wolf Trap Opera Company is a versatile, fun-loving young crew, if we can judge by what happened last night in the Barns. Two one-act comedies by Rossini and Donizetti brought the audience close to terminal fits of laughter. Three more performances, tonight and Aug. 22 and 24, should play to standing-room audiences.

Rossini's "Il Signor Bruschino" is a situation comedy barely distinguishable from the contemporary product except that the music is a lot better. Donizetti's "Le Convenzione Teatrali" (or "Viva la Mamma") is a tensely funny drama of backstage intrigue at an opera house. Ten soloists performed with distinction, including seven who took more than one role. There were two conductors; Stephen Sulich, who conducted "Il Signor Bruschino," joined the cast for "Viva la Mamma," while Richard Woitach took over (splendidly, as usual) in the pit.

The music of Rossini and Donizetti was fine, but it was upstaged by two items used as audition pieces: Bellini's "Per pieta , bel idol mio," in a breathtaking performance by Victoria Livengood, and Mozart's "Der Ho lle Rache," an aria whose high notes terrify most sopranos, sung . . . well, not entirely sung, but at least performed, by baritone Robert Mattern.

It is fairly common to see women singing men's role in bel canto opera. Mattern struck a blow for equality by singing the role of Donizetti's "Mamma,"a fiercely ambitious backstage mother who aims to make her daughter a star through intimidation and willingly takes on a role herself after scaring off the originally scheduled singer. For the Mozart aria, he sings, talks, grunts, chirps and whistles in a voice that spans about four octaves, give or take a few microtones. His tone-production is not completely even throughout that range, and his diction suffers above the treble staff, particularly when he whistles. But he does produce a remarkable variety of sounds and when he reaches for a note, he gets it -- more or less.

Musically, Livengood's was the most impressive voice of the evening, but soprano Rachel Rosales and tenor Richard Croft were both outstanding, vocally and dramatically. So were Richard Paul Fink in the title role of "Bruschino" and James Michael McGuire as the composer in "Viva la Mamma." A special challenge was faced by soprano Barbara Kilduff, who was required by the plot of "Viva la Mamma" to portray a soprano of minimal abilities. Fortunately, she had already shown a superb voice in "Bruschino." Major credit for the evening's intensely comic impact goes to the inspired stage direction of Francesca Zambello.