They were all wet at Wolf Trap last night, and as the chatelaine of the place put it later, emerging from her plastic dropcloth, it added a new dimension to things.

The downpour caught some 600 formally clad gala-goers out in the open after they had paid anywhere from $160 to $250 each to be in line at the opening of Wolf Trap's 1979 summer season.

There were those who said they had believed until now that Kay Shouse, Wolf Trap's donor, possessed some magical power to keep the rainclouds hovering but never opening. Mrs. Shouse, it was believed, consulted her farmer's almanac to verify what she already knew: it wouldn't dare rain at her gala.

Except that this year it dared. Skeptics in the crowd who had brought along umbrellas popped them open and ate their beef stroganoff, poached salmon and spinach mousse alfresco under the raindrops.

The unprepared either cowered under trees on the soggy grass or huddled inside the Wolf Trapper Shelter.

"I'll never trust the almanac again," said Shouse.

"A slip of the angels," consoled the wife of the Brazilian ambassador, Mrs. Joao Battisto Pinheiro.

"I love it," said the wife of the Japanese ambassador, the eternal diplomat Mrs. Fumihiko Togo. "I dry very quickly."

Shouse's head table had taken on the appearance of a Noah's Ark, stranded as it was out in the middle of the lawn. Among those sharing it with her were Edward Villella, Virginia Lt. Gov. and Mrs. Charles Robb, Rep. and Mrs. Joseph L. Fisher (D-Va.) and Fairfax County Supervisors Jack Herrity and John Shacochis.

"Some of us are dry on the outside but not on the inside," Herrity said.

"I am sure Jack wouldn't want it known he was dry on the inside," said Robb.

"God was just sprinkling a little holy water," said Shacochis.

Skies had looked threatening when the guests arrived around 6:30 but except for a few sprinkles the rain had held off nearly an hour. Huge tents protected the buffet tables and bars, but dinner tables were out in the open.

The striped Angelo Donoghia sheets doubling as tablecloths and the pink napkins matching the floral centerpieces were anything but drip dry and so were the Oscar de la Rentas, the Bill Blasses, the Christian Diors. People without plastic wrap or umbrellas draped their pink napkins over their heads and went on eating and drinking their Virginia vintage Villard Noir by Meredyth Vineyards of Middleburg.

"I've a feeling we're going to leave early," said Washington patron of the arts Sidney Zlotnick. "There's no way we can get the chauffeur," moaned his wife Evelyn, who was wearing a Paris original.

"I'm glad I only paid $25 for this jacket," said a young woman starting to wring it out.

Part of the evening's game became taking count of who was dry and who was wet. The American Film Institute's George Stevens and his wife Liz were dry, having arrived late because of a flat tire. Sen. Richard Schweiker (R-Pa.) and Sen. William Cohen (R-Me.) were also dry. Democratic National Chairman John White threatened to crawl under a table and former Housing and Urban development Secretary Carla Hills was gallantly sheltered by an umbrella her husband Rod held over her head. A combo with a sense of humor struck up "Singing in the Rain," but what else was there to do but sing, somebody said.

Even before the rains came, people were wondering if the evening might be a washout - but because of talent, not weather. Opera singer Shirley Verrett had pulled out at the last minute as one of the evening's performers (felled by a liver ailment contracted in Italy, said a Wolf Trap official), and so had pop singer Neil Sedaka (contractual problems in Canada).

"The highlight of the evevening," ventured public relations expert John Meeks, "will be the intermission."

The gala made $150,000, or $50,000 less than last year, but a respectable take, said Martindale, who had been ticket chairman in 1978. Anna Moffo, the opera singer, was the 1979 general chairman, but had a hard act to follow after two years of Elizabeth Taylor at the helm. Taylor brought in names like Sammy Davis, Jr., and Liza Minnelli in 1977 and Bob Hope the following year.

By the end of the evening yesterday, the sky was dry and the air fresh and even the audience had a chance to "star" in the show. When the Brazilian ambassador introduced a lively troupe of carnival dancers, even master of ceremonies Edward Villella, a principal with the New York City Ballet, couldn't resist getting in the act.

Elizabeth Taylor returns next year to chair Wolf Trap's 10th anniversary gala and according to officials she is already lining up the talent for it. CAPTION: Picture, Beaton Tolley, left, and Mrs. Joao Battisto Pinheiro share an umbrella, while Virginia Lt. Gov. Charles Robb, right, ducks under a plastic sheet; by John McDonnell - The Washington Post