When the National Gallery of Art closed the doors on "The Treasure Houses of Britain" Sunday night for the last time, there were no royal visitors, glittery guests or gallery officials. The gallery employes on duty did notice that one of the last visitors was an unidentified white-haired woman, whom they recognized because she had visited the show a number of times during the more than five months that it was here.

The gallery estimates that more than 990,000 visitors saw "The Treasure Houses of Britain" since Prince Charles and Princess Diana and a bevy of social and political celebrities flocked to an array of social events in November at the exhibition's opening. It outdrew the "King Tut" exhibit, the previous record holder with 835,000 visitors. "Treasure Houses" had the advantage, since it ran about three weeks longer than Tut. The Tut exhibition wasn't as well organized for heavy traffic, either. The galleries were overcrowded and visitors often had to wait in line for as long as five hours to see it.

"Treasure Houses" brought so many people to Washington that it became a "must" tourist attraction for celebrities and other notables. British Ambassador Sir Oliver Wright joked that from the number of royal and other visitors at the embassy, "we were able to keep the sheets changed, but the beds never cooled down." Where He Went, It Was Michelob

Japanese Prime Minister Yasuhiro Nakasone, either uninterested or unaware that Sunday was the last opportunity to view "Treasure Houses," visited the National Gallery of Art that same day for a special tour of the Winslow Homer show conducted by Gallery Director J. Carter Brown. After viewing the Homer watercolors, Nakasone, who is also an oil painter, went upstairs to the permanent collection to look at some of the Winslow Homer oils.

Later that evening, Nakasone and nearly 100 others, including Japanese Ambassador Nobuo Matsunaga, ate at the Evans Farm Inn in McLean. The prime minister wanted a typical American dinner, so he was served roast duckling with orange sauce, wild rice, green beans with toasted almonds, and lemon sorbet. They were served California wine and Michelob beer. With the party, however, were about 50 security people from the State Department, Secret Service, Fairfax County Police and Japanese security. His typical American dinner was attended by a bomb squad, Dobermans and marksmen on the roof. End Notes

An honorable man never kisses and tells. Film director Roger Vadim, who has loved some of the most beautiful women in the world, has written a book perhaps telling a bit too much, and two women who were an important part of his life are suing. Actresses Brigitte Bardot, to whom he was married, and Catherine Deneuve, with whom he had a son, are seeking damages over his book "From One Star to Another," saying it intrudes into their private lives. A spokesman for his French publisher said the book was a "friendly, sweet account" of his life with the women. Vadim was also married to Jane Fonda. The English-language edition of his book is titled "Bardot, Deneuve, Fonda" . . .

Cartoonist Garry Trudeau, whose popular "Doonesbury" cartoon, which appears elsewhere on this page, is in trouble again. The Los Angeles Times, no less, is pulling his irreverent strip this week. While saying it was "distasteful" to interrupt a regular feature, a spokesman from the Times said the paper was doing so because "we feel this week's 'Doonesbury' grossly exaggerates the real and alleged transgressions of many Reagan administration appointees." Trudeau, who generally does not speak to the press, had no comment. A spokesman for the Universal Press Syndicate, which distributes the strip, was unconcerned and said the Times was the only one of the strip's 880 papers to take such action so far . . .

The hospital beat: Adlai Stevenson III has been having his problems. The former Illinois senator and current candidate for governor was thrown from a horse Sunday and is being treated for a fractured vertebra. He had been riding with his daughter, Katie, on his northwestern Illinois farm when the accident occurred. A spokesman described the injury as "a nuisance fracture" that would not require surgery or traction. Stevenson, who is challenging incumbent Gov. James R. Thompson, is considering running as an independent candidate because supporters of political extremist Lyndon LaRouche won the slots for lieutenant governor and secretary of state on the Democratic ticket . . . Sen. Barry Goldwater is described as being in excellent condition following hip surgery at Walter Reed Army Medical Center. The 77-year-old Arizona Republican underwent surgery over the weekend to remove wires used in previous hip replacement operations that had worked their way into the surrounding muscles and nerves. He is expected to remain at Walter Reed for 10 days.