Nowhere in official Washington is the line between governmental duties and private favors blurred with less regret than in the realm of art.

That may partially explain why an official-looking truck, a white one marked "National Museum of American Art, Smithsonian Institution," was yesterday afternoon dispatched to a private house in Georgetown. The truck and its four passengers -- James Volkert, the museum's chief of design, and three of his assistants -- had been sent to do a favor for the woman who lives inside.

Nan Tucker McEvoy had a small problem. A work of art she owns, a sculpture, was not in the right place. It needed moving in her house. The sculpture, of cast bronze, is neither large nor heavy. Its limestone base, however, weighs 700 pounds.

She might, of course, have summoned a bunch of beefy friends or commercial movers. But McEvoy has given time as well as money to the National Museum. For seven years she has been a member of its advisory commission, and on Jan. 1 she completed four years as its chair. So Volkert and his crew did the job instead.

"They did the work at my request," she said yesterday. "I called Charley Charles C. Eldredge the National Museum's director and asked if he could help. I only wanted it moved 20 feet, but there was no way in the world I could get it into place myself. I thought technical skill was required. It is not the sort of job you assign to boys from Georgetown University."

The job was finished in less than an hour. Volkert says his efforts were "appropriate and ethical." And his boss agrees.

"We were merely providing modest assistance to a friend of the museum's," said director Eldredge. "Her sculpture, by Bryan Hunt, is a study for a larger piece we've considered for acquisition. We may someday wish to borrow her Hunt for exhibition. Under the circumstances, it seems to me, we did nothing wrong."

Though one museum employe says that "all our jaws dropped when we heard of the request," Eldredge's superior does not seem upset.

"It is a judgment call, I admit," said Smithsonian undersecretary Dean W. Anderson who, among his other duties, is temporarily serving as assistant secretary for history and art, and as such is charged with supervising the Smithsonian's art museums. "As a general proposition it is worthwhile trying to be helpful to people who, in the future, may be helpful to you."

The Hunt sculpture the museum is considering is a cast bronze object nearly 12 feet tall. Its title is "Stillscape II." Hunt, a 38-year-old New Yorker, plans an edition of three. Two have been cast so far. One of these was displayed in last year's Whitney Biennial exhibition. Though he leaves such decisions to BlumHelman, his Manhattan dealers, Hunt said yesterday that "Stillscape II" would probably now sell for between $100,000 and $150,000.

Not all national art museums are so helpful to their friends. "If some donor to the National Gallery of Art asked me to hang pictures in their living room," said Gaillard F. Ravenal, head of the design staff there, "I'd tell them to suck eggs."

Though the National Gallery's curators, art handlers and conservators are often asked for favors -- be it appraising the market value of a painting, repairing a frame or cleaning an engraving -- a longstanding gallery policy prohibits them from doing so. All exceptions must be cleared by the director, and even in such cases -- say, when a work of art has been pledged to the museum -- a professional fee is charged.

McEvoy, however, was not asked for a payment. But she does feel that her Hunt sculpture is "of museum quality."

"My guess," she said, "is that someday it might go to the National Museum of American Art."