We've all done it: On your vacation, you walk into a post office in some town you don't know -- and there is this great mural that sprawls the length of the lobby, boiling with action and heroic figures.

And you want to know: What is this wonderful thing doing in Rushville, Illinois?

The natives don't even notice it anymore. "Oh, it's been there since the '30s," someone says.

On Friday the National Museum of American Art will open a show of post office murals and other public art from the New Deal period. There are 58 studies at one-sixth scale, eight sculptures and two actual murals, and they are about as close as America has ever come to a true people's art.

Those were great days for murals anyway, with Diego Rivera and Thomas Hart Benton in full swing, and even Lee Krasner created one of her swooping flower fantasies for a Manhattan lobby. But there was to be no abstraction here, no avant-garde experimenting, for the federal government was paying the bill, and the artists were urged to consult with community leaders about their works.

The Section of Fine Arts, founded in 1934 in the Treasury Department, which is in charge of decorating public buildings, was not to be confused with the Works Progress Administration, a New Deal program for out-of-work artists. The competitions were open to all, and in nine years about 1,400 commissions were awarded -- to the likes of Henry Varnum Poor, Boardman Robinson, Stevan Dohanos, Guy Pene du Bois, Rockwell Kent, Philip Guston, William Gropper and Peter Blume.

There are a lot of Indians in these pictures, a lot of Western vistas, assorted Founding Fathers and noble workers. Cousins of the folkloric farmers of Thomas Hart Benton, gnarled and bent and slightly warped, appear again and again, and here and there is a touch of Reginald Marsh sketchiness, the storybook stylization of Rockwell Kent.

But the main thing is, this art has liberated itself from the traditional allegory and pomp of murals. It has an outdoor feeling. You can smell the air. This is America's view of itself at a time when the entire proposition of democracy was in some question, and cosmic doubts hovered over Wall Street and soup line alike.

It is a cheerful and romantic view.

The Knoxville, Iowa, post office mural depicts an 1845 land rush when, according to the local newspapers at the time, "covered wagons drawn by oxen rattled over the roughground. Shouts of excited drivers mingled with the yells of men on horseback carrying torches, flowing in the wind ..."

Jenne Magafan's "Cowboy Dance," a study for the Anson, Tex., post office mural, swirls with flying skirts and kerchiefs. You can hear booted heels on floorboards. William Gropper's dramatic "Construction of the Dam," for the Interior Department, flows in great waves of action, breaking on the mountainside and the dam's ribs at opposite ends.

The heroic workers in "Riveters," a strong relief sculpture by Chaim Gross, could be almost a signature for all New Deal art. A lovely wood relief of a cow and calf by Boris Gilbertson -- a high point of the show -- was never used, for by the time it was completed, late in 1943, the program had been cut short by the war.

"In view of war conditions and the urgent need for conservation of funds and material," wrote the postmaster of Macomb, Ill., "there would be a pronounced feeling against the project in this community." Too bad, considering the thing was already done.

The role of local citizens seems to have been rather admonitory at times. They knew what they wanted, and they weren't about to be dictated to by some bearded guy with painty shoe tops.

Wendell Jones sketched a magnificent scene showing the people of Cairo, Ill., filling sandbags and building a levee to save their city from the Mississippi River. They had told him proudly that "not a drop of water ever got into the streets," so his mural had homemakers and bankers, stevedores and clerks, black and white, working together.

When they actually saw "Sandbagging the Bulkheads," however, the people of Cairo decided that they didn't want to be reminded of this perennial threat. Possibly the chamber of commerce was looking over its shoulder. They refused to have it put up, and later the mural disappeared, as murals will, no matter how big. Only the study is left today.

All these works, by the way, come from the museum's permanent collection, according to Virginia Mecklenburg, curator in charge, Department of Painting and Sculpture.

"Some of these pieces have been here since 1962," she said. "The studies were government property, of course. We found one 400-pound plaster relief in an attic at Interior. It was in a crate, and we went up there with flashlights and discovered it was a full-scale model of a relief for the post office at Ossining, New York."

Apparently the local postmaster had been nervous about having a 400-pound object hanging over his door. He sent it back, and no one in Washington knew quite what to do with it.

Not all the pictures are group scenes. Stevan Dohanos painted six panels from the life of James Hamilton, a mail carrier in Florida. Hamilton's route took him barefoot along the beach from Lake Worth to Miami. One dark day in 1887 he was attacked by alligators and killed in the line of duty.

Another study shows an abolitionist farmer hiding escaped slaves at the Brockett Farm near Dolgeville, N.Y., a Mohawk Valley station on the underground railway.

Despite its limitation to "past or present history," the show, which is called "Special Delivery: Murals for the New Deal Era," manages a variety of styles. A Santa Monica, Calif., library mural combines an expressionistic dragon with early humans making fire. Woodrow Crumbo's "Buffalo Hunt" is almost Japanese in its ornamental figures and plants. An Indian battle scene, Illini versus Potawatomi one of the few warlike pictures here, verges on the abstract.

At the other end of the scale is Peter Blume's charming study of "Beatty's Barns" for the Canonsburg, Pa., post office. A winter scene, abandoned barns, it has a spare, contemporary look, as cleanly detailed yet romantic as a James Wyeth. Lyrical touches -- a bare-branched tree, a fence wire -- take it beyond the literal, so that it is evocative but not painfully so.

Most of these murals were six or seven feet high and about 13 feet long, and the artist could expect to be paid slightly more than $700 for one. Many are still in place, no longer really seen, perhaps, by the people who pass them daily, but always ready to astonish a newcomer.

Optimistic and even quaint they may be, reveling as they do in our homespun history as we would like to imagine it, with peaceloving Indians and sculpturesque steelworkers, fecund farmlands and post-card mountains. But it is good to be reminded that this was in fact how we saw ourselves in the bottom of the Depression, and that some of it anyway is still true.

George Biddle, a former schoolmate of Franklin Roosevelt, suggested the program in the first place, using taxpayers' money to buy public art. He told FDR it would create "living monuments to the social ideas that you are struggling to achieve," and Roosevelt agreed, while warning him not to allow anything too specifically like New Deal propaganda.

The show goes on until Sept. 11. It's a joy. Maybe it will even get a few people to look up at the walls next time they go to buy a stamp.