One day three years ago, Ralph Carpenter, who was then Texas Tech's sports information director, declared to the press box contingent in Austin, "The rodeo ain't over till the bull riders ride."

Stirred to top that deep insight. San Antonio sports editor. Dan Cook counteres with, "The opera ain't over till the fat lady sings."

When the San Antonio Spurs were down in a basketball chamionship series with the Washington Bullets recently Cook repeated the phrase on the evening sportscast, television and thus set off a chain of events that have led it to become a sort of motto for the Bullets.

Enter Bullets coach Dick Motta, who heard the "fat lady" comment and thought it funny enough to repeat it himself , during another basketball game in Philadelphia, cleaning up the language along the way. (Motta doesn't use languge like "ain't says Bullets marketing chief Charles (Chip) Reed, who had considered promoting the slogan on bumper stickers and pins in the Bullets' curent contest for the national championship with the Seattle SuperSonics.)

Enter Alvin Meltzer, T-shirt entreprenuer who read the phrase (complete with "ain't") in the paper three weeks ago. His firm in Hyattsville, The Shirt Explosion , makes rock star T-shirts and other publicity items that help people advertise while getting customers to pay for it.

Enter a sleepy art director Alfred DeVita, waked up in the morning by Meltzer to start work on a possible T-shirt featuring the Fat lady slogan. So sleepy was DeVita that he thinks he heard Meltzer say "isn't," not "ain't."

Say "opera" to DeVito and he thinks of "Die Walkure" and hucky blond prima donnas sporting head-pieces and shields.

Two days later the proposed T-Shirt, complete with forged Motta signature, was in the office of the Bullets marketing man, Reed. The next day, with authentic signature fromMotta in place, the Shirt Explosion started to print polyester and cotton crew neck Fat Lady shirts, turning out 840 pieces per hour.

Melter experts that 1,000 dozen shirts will be sold at $5 apiece before the championship series is over. Not as good a seller as rock star T-shirts perhaps, but far better than, say, another current popular style from Shirt Explosion, "Don't but books by crooks," created for a group to inhibit sales of Richard Nixon's memoirs. (Among the big purchasers of Fat Lady T-shirts are fans in Portland, Ore., home of Seattle's archrivals)

Enter fat ladies. The Bullets have received drawings of the from elementary school children in Prince George's County. A University of Maryland sorority has offered to conduct a search for an appropriate model. And at least three dozen fat ladies have called the Bullets office volunteering to be the Bullets' symbol - in order to get into the games.

Enter the "mystery fat lady," to be revealed during Sunday's game here. One day last week a Walkure lookalike appeared at the Bullets office complete with opera credits. The Bullets won't reveal her name but she'll be decked out with shield and spear for the game. "It's amazing," says Reed, "what people will do for two free Bullets tickets."

Motta received a token sum for the use of his name. The Bullets will make 70 cents from each sale. The shirts are available at the games and at several local stores.