Washington's Gay Men's Chorus made its concert debut last night to a full house of enthusiastic supporters at St. Mark's Episcopal Church on Capitol Hill. For a group that has been together only six months and less than half of that with its new director, Nick Armstrong, it could be counted a promising beginning.

Armstrong has already made commendable strides toward shaping a warm, resonant sound and establishing clear diction and well-defined melodic lines. As he continues to work with the chorus, its tone and balance should become more even--added weight on the bass end would be particularly welcome--and the intermittent pitch problems most likely come under control.

No doubt the programs will also become more focused. On this evening Armstrong tried to include something for everyone, and that may be all right, even necessary, the first time round. But if the chorus wants to build a steady following some hard choices will have to made. Song and dance routines, inside jokes, folksy turns and pure classical selections, however pleasing individually, make for musical indigestion when mixed together.

The program will be repeated tomorrow night at 8:15 at St. Mark's and Saturday at 8 at the First and Franklin Street Presbyterian Church in Baltimore.