BETWEEN RHYTHM AND BLUES, if she had to choose, Cathy Ponton would probably say the biggest problem these last four years has been the rhythm. Not the concept, but the section.

Bass and drums, in other words.

Things are fine at the moment, though -- or as fine as they can be when the band's van is in the shop and not as many folks go out to hear blues in Washington as they did when Desperado's was around. The Rhythm Masters -- four years and approximately 729 rhythm sections after singer-guitarist Cathy Ponton and harmonica player Pat Day started the band in 1980 -- have survived yet another turnover.

And this time the drummer can sing.

As for Ponton's voice -- well, you may have heard it on the radio some years ago (she used to do newscasts for WMUC at the University of Maryland, WLMD in Laurel, and later ABC Radio), but you haven't heard it until you hear it in the middle of something by T-Bone Walker. Her voice, shaped amid a primarily musical Irish family during Ponton's Hyattsville childhood, is nowadays as sharp-edged and snarly as Bonnie Raitt's -- though if you tell her that she may worry about being typecast. (Throw in a Billie Holiday or an Aretha Franklin and she'll feel better.) Ponton also plays guitar, after the likes and licks of John Lee Hooker and Albert Collins.

In other words, you would have to be either truly crude or deaf if the only compliment you could muster after a Rhythm Masters show concerned Ponton's black leather skirt and white pumps. Perhaps you should just say something nice about the guys -- whose ages, like Ponton's, hover around 30, and who are not just doing this for their health, you know.

Bassist Russ Dunlap, for instance. An electrician in Gaithersburg most days, Dunlap joined the band early last summer, around the same time drummer Big Joe Maher arrived. Maher arrived with a big voice and enough loose energy to enable him to still play in the house band at the Gentry plus about six other bands, when he has the chance. And Day himself, who can play just about any harp solo you may have heard in Chicago, is the only band member who lives mostly on band income (except for some occasional carpet-installation work). And Day handles the band's bookings, which span the Carolinas to upper New York state.

"The band doesn't really make any money," says Ponton, who supports her own music habit by working whenever possible at a savings and loan on Wisconsin Avenue. "It's all reinvested in promotion, traveling and phone bills."

Base pay per man per night, Ponton says, is probably about $50 -- though it can get as high as $150 on a good night. "But we have so many overhead expenses. That explains why a lot of bands break up, and give up. It's just unbelievably expensive to run a band. I was very naive when we started. I have learned a lot."

These days look better, though -- what with Big Joe around, Ponton and a singing-songwriting friend nearing completion of a 16-track recording studio in Vienna, and the band averaging about 15 jobs a month (12 of them on the road).

Plus, of course, the occasional Good Night -- like the one last week in Rehoboth at the Country Squire, or the one last year at the Wax Museum, when headliner Albert King invited Ponton onstage to sing "You Got Me Runnin' the band.

"After those kind of things happen," says Ponton, "you feel great, you forget about the van breaking down, or somebody leaving the band, or a club that closes, or all the things that are discouraging about it." THE RHYTHM MASTERS -- Appearing this Friday-Saturday at The Barn, Parkville, Md. (near Towson); January 22 at Prince George's Community College, Largo; January 24 at the Bayou with Roomful of Blues and February 2 at Friendship Station, 4926 Wisconsin Ave. NW.