Frances Thompson McKay grew up thinking that "improvisation" was a dirty word in the world of classical music. Beethoven did it. Mozart did it. But the message sent to McKay was: "It's not something you do!"

The forbidden has a haunting appeal, and now that McKay has grown up, she does as she sees fit. She improvises.

For the past several years, McKay, a pianist and composer, and Katherine Hay, a flutist, have been improvising together twice a week, Monday and Friday mornings. "We were having such a good time playing together, and we began to think it was so good that we decided it would be fun to go public with it," says McKay.

Hence, Music of the Spheres at St. Mark's, Washington's newest music series, with improvisation as its motif, and McKay and Hay its artistic directors. The four-concert series, which begins tonight at St. Mark's Episcopal Church on Capitol Hill and continues through June 1, will put the disciplined freedom of improvisation back into classical music.

"There is a big gap in today's concert world, as far as improvisation is concerned . . . You don't see people get up and improvise right off the top of their heads in a western, classical style . . . and I think this series helps to fill in that gap," says McKay.

The gap seems almost absurd at moments. According to Hay, " Juilliard has a class -- for actors only -- in improvisation. A class in musical improvisation for actors. But no class in musical improvisation for musicians."

Improvisation comes in all forms and degrees of inventiveness, but the kind most likely to be in the series, says Hay, is "improvisation with a particular musical structure -- with patterns written out, or rhythmic motives written out -- which the performer then plays around with, develops."

How does improvised classical music differ from prepared music when it reaches the audience?

"I think there is a certain excitement for the audience to know that this person is on his own personal journey," McKay says. "There is an added intensity to any improvised performance, because you are bypassing one of the stops on that triangle between composer, performer, audience. Usually we go from the composer, through the performer, to the audience, but here the composer and the performer are one."

Tonight's concert reaches back to the medieval world for its improvisational momentum. Scott Reiss and Tina Chancey of Hesperus and the Folger Consort will present medieval melodies, American colonial dances (an anachronism they enjoy) and an improvisation on a tape-recorded arrangement of a medieval estampie.

Chancey and Reiss are already expert improvisers -- it comes with the territory of medieval music, where the performer often has little indication about how or on what instruments to play the music. Classical and modern composers are a lot fussier about details than medieval composers were. Medieval performers had a lot more freedom of choice -- and responsibility for the results.

"We finally get a chance to let it rip," says Chancey of the opportunity the concert offers. "We can give free rein to our improvisational urges without having a feeling of educational responsibility . . . In standard concert situations, we are careful to be as historically authentic as we can without sacrificing the joy of the music."

Improvisation, says Reiss, "takes you out of the realm that is safe. You are on the spot, demanding of yourself an instantaneous creation that has to be good. That's a much different kind of challenge than performing impeccably prepared music."

And whatever improvisation might lack in depth of interpretation or technical polish, it makes up for in immediacy and freshness.

"It is like watching somebody walk a tightrope," says Chancey. "The way it is set up, the audience will know we are there to take chances. If we fail, we will fail grandly, and with great enthusiasm."