The hosts of the gilded gala at Pisces in Georgetown last night were the Baron Hans Heinrich Thyssen-Bornemisza and his wife, Denise. The guest list of about 130 was a blend of art and politics--heavy on the art.

Even the British Ambassador to the United States, Sir Nicholas Henderson, was able to take time out of his busy schedule to sample the salmon and tournedos. How could he do it?

"I won't be here long," Henderson said curtly, and he turned back to his dinner conversation.

Attorney General William French Smith also was one of the more politically oriented guests of the evening. Had he talked to President Reagan lately about taxes?

"I never comment on matters like that until I've had two drinks," Smith said with a smile, looking to the white-gloved waiter who presented him with a glass of white wine on a tray.

The women in long gowns of gold and black picked up their dresses as they walked gingerly down the spiral staircase, holding on to their escorts who wore tuxedos, before reaching the receiving line made up of the baron and the baroness. In the background, Peter Duchin's orchestra played swing tunes.

Many guests stopped to look at what appeared to be an original oil painting of bright red and orange flowers by Emil Nolde, which hung next to the curve in the stairs. The real flowers at the base of the stairs were a perfect match for those in the painting.

"Flower Garden," a reproduction of a painting in the Thyssen-Bornemisza collection, which hangs ready to open at the National Gallery on Sunday, was lighted and hung against a black curtain by the stairs.

Among the guests were International Communication Agency director Charles Z. Wick; Swedish Ambassador Wilhelm Wachtmeister and his wife, Ulla; Secretary of the Smithsonian S. Dillon Ripley and socialite Evangeline Bruce.

Why did the baron choose to throw the lavish party? "Gee," said Frederick Cody, vice president of Indian Head, one of the baron's companies, "he owns the pictures, honey. Isn't that enough?"

J. Carter Brown, director of the National Gallery of Art, was one of the guests who had been with the baron the night before at the party celebrating the opening of the exhibit. At that party, the baron commented he did not plan to donate any of his spectacular collection to the National Gallery. Brown explained last night that it was not unusual for a private collector such as the baron--who is from Switzerland and is a legal resident of Monaco--not to donate works to a museum in America.

"It doesn't surprise me," Brown said. "He doesn't pay any income tax here, so there's not much incentive to donate."

Many of the art connoisseurs at the gathering were just happy to have the exhibition temporarily.

One of them was collector David Lloyd Kreeger. "This is his achievement," Kreeger said. "There aren't many people who know how to use great wealth to wide enjoyment."

One guest, however, did not share Kreeger's enthusiasm. William Draper, president of the Export-Import Bank, had mixed feelings about the new exhibit.

"I think the art in this show is strong," Draper said, arching his dark eyebrows, "like the baron, who is a strong man . . . but it's not the Rodin. I miss the Rodin."