"I think we should change the music from grand opera to 'Singing in the Rain,' " said designer Oscar de la Renta as he watched the rain gush between two adjoining tents set up in the garden of the Embassy of the Dominican Republic last night. And as the waiters tried to seal the gush, the water poured onto the white-sheeted runway.

"We'll see if the clothes are waterproof, won't we?" joked the Dominican Republic-born designer as the music cued the models to appear.

Last night's de la Renta showing, a black-tie event, was the first of many fashion shows this fall. And as a benefit for the Washington Opera, it was expected to raise more than $25,000, according to Renee Kraft, one of the chairs of the event.

If the models weren't dressed for a Washington downpour, they were certainly prepared for a Russian winter -- with fur-lined, quilted leather coats, sable hats and suede boots.

"Pure Boris Godunov," Martin Feinstein, the opera's general director, commented from a ringside table, as models swept over the runway surrounded by dinner tables where 200 guests turned their chairs to watch the show.

"We have lots of rain in the Dominican Republic, and we think it is good luck. It brings abundant crops," said Dominican Republic Ambassador Rafael Molina Morillo. Better luck still, the rain stopped before Feinstein could say Rimsky-Korsakov and tile print silk dresses appeared on the runway.

Honorary patron Susan Baker, wife of President Reagan's chief of staff, James Baker III, checked out the de la Renta taffeta harem pants before the show. "One never wears pants to the White House," she told the designer, "but I think I could wear these."

Jean Smith, who has attended all the hearings on Supreme Court nominee Sandra Day O'Connor, arrived with her husband, Attorney General William French Smith. "I'm not sure he even knows where he is," she said. "What's going on tonight?" the attorney general asked, and was clued in on the event.

Jean Smith was wearing a de la Renta dress with black velvet bodice and emerald-green skirt. "It's the easiest dress I own and so I wear it all the time," she told the designer. "I just zip it up and I'm fully dressed." Aside from one new Oscar for the season ahead, she hasn't bought another thing for fall. "It's nice because no one here knows what I already own."

Buffy Cafritz and Francoise de la Renta wore similar versions of the same paisley silk dress. "I sent the maid out to buy a clean shopping bag for 25 cents, put the dress in the bag and arrived without a wrinkle," said the designer's wife, who was wearing a heavily encrusted belt and several necklaces, and who flew down from New York with her husband and two models this afternoon. "You must be strong to wear all this jewelry," she teased. "But I've had years of experience."

De la Renta had suggested to Bloomingdale's, which sponsored the benefit, that it be held at this embassy. "The ambassador is new here and the Dominican Republic has a tough time in a city with a zillion embassies." De la Renta left the Dominican Republic at age 17. "I never went back to my country for 11 years. We had a dictator, and I never knew if I could get out again."

He goes back often now. "I don't relate work to my country -- I never work there," he said. About his current collection he commented, "I always think grand and I happen to love the opera and so the tie to the evening is a natural one."