Every Tuesday night from September to June, 185 Washingtonians forgo leisurely dinners, "Roseanne," "thirtysomething" and early bedtimes just for the sake of a blast of Bach or Rachmaninoff. They'll even take off from work and dress in penguin suits or regulation royal blue gowns. None of them will make a cent from any of this, but the members of the Choral Arts Society of Washington couldn't care less.

Tomorrow evening, the society will perform its 25th Anniversary Gala concert at the Kennedy Center Concert Hall, hosted by music critic Paul Hume and featuring luminaries such as Mstislav Rostropovich and pianist Eugene Istomin.

"The existence of this wonderful group," raves Rostropovich, "has completely changed the life of Washington, D.C. This is because they are genuine people who perform out of love for the arts, a love of beauty and a love for Washington."

"It isn't just the music. It's also a spiritual thing," says alto Izzie Betts, who has sung with the group since its inception 25 years ago.

"Obnoxious egos don't appear in this chorus. It's an absolutely joyous group to be in," declares second bass Doug Gill, a 17-year veteran and field biologist.

No matter how many times one has heard the Choral Arts Society perform, one cannot help but marvel at the perfect blend of voices, the uniformity of tone and diction. Ask any of the society's members why they sound the way they do, and they all respond in kind -- it's the doing of conductor Norman Scribner.

"Norman treats us all as professionals -- unlike other conductors, who are often condescending in some way," says Gill. "After 17 years, I'll still go home after a rehearsal and tell my wife that he taught me something new."

"I just like his disposition so much!" exclaims Bertha Donahue, a composer, violist and one of two female tenors in the chorus. "I once sang in a chorus where the director had the worst temper. He'd throw things and say, 'You do not deserve this music.' He even sent somebody out once. There was fear all around. But with Norman, even if we act up a bit, he's really nice about it."

"He's constantly rearranging the seating chart," says Betts, "trying to get the voices to blend better. And he has a great sense of humor."

That mixture of precision and intensity for which Scribner is known harks back to his early years as a keyboard player and choirmaster. The son of a Maryland clergyman, he spent his high school years racing to the church at 4 a.m. each morning to practice scales. After his father died, financial necessity compelled him to focus on more lucrative forms of music making: church jobs. So perfection-oriented was he that he held sectionals for a junior choir and rigorous auditions for his Baltimore Choral Society, formed while he was still a student at the Peabody Conservatory. (Even now, he requires each member of the Choral Arts Society to re-audition every year.)

In 1960, after a stint in the Army, Scribner took a position at the Washington Cathedral as musical staff assistant, choirmaster of St. Alban's Church and chapel organist for the St. Albans School for Boys. Since then he has immersed himself in the musical life of this city, forming his own small group -- the Norman Scribner Choir -- composing, serving on the NEA choral music panel and, of course, creating and molding the society.

In those years, the group has been hired to participate in such emotion-laden occasions as the World Holocaust Convention, the funeral procession of Robert F. Kennedy and last year's choral tribute commemorating the life of Martin Luther King. Maestros like Leonard Bernstein, Erich Leinsdorf and especially Rostropovich take pleasure in working with these particular singers. The great Russian exile led the group in a landmark performance of Rachmaninoff's ethereal "Vespers" at the Washington Cathedral, the first performance ever by an American chorus.

Scribner shrugs off accolades. It's not him, he insists, it's the "heady atmosphere" created by this collection of "vibrant individuals." Anyway, he'd prefer to talk about his goals for the future -- more commissions from American composers, more broadcast opportunities, more recordings (the Choral Arts Society's performance of the "Vespers," recorded and released by Erato records, won the group international acclaim; it'll record a selection of Christmas music in July) -- as well as his attempts to modernize his organization.

"The sands of the concert world are shifting under our very feet," he says. "Tastes are constantly changing. There is always some wrinkle to keep up with. And the arts are getting more and more expensive. We have to be adaptable." To this end, the society, long a mom-and-pop operation, has metamorphosed into a highly professional enterprise. That includes a full-time administrative staff, a new and very active board, an endowment campaign, computers in the office, and a program devoted to minority participation in choral music in Washington.

All of this "institutionalization," as Scribner calls it, doesn't mean that the chorus will be abandoning the traditions and repertoire represented, in excerpts, on tomorrow's celebratory program: Brahms's "A German Requiem," Bach's Mass in B Minor, Orff's "Carmina Burana," Bernstein's "Chichester Psalms," the Rachmaninoff "Vespers" and other audience and chorus favorites.

"It's certainly been a very full 25 years," muses Scribner. "I can remember with crystalline clarity those first concerts, and it's still a most refreshing and uplifting thing to be able to do. We're striving for something that's larger than all of us."