Where folk art and bric-a-brac rub elbows with Rembrandts. Her parents collected Manets and Vermeers. But Electra Webb, co-founder of the Shelburne Museum in Vermont, liked Americana. Now, part of her collection is on view at the National Gallery.

Should a large wooden rooster wearing a saddle and stirrups be considered in the same league as paintings by Monet, Georgia O'Keeffe, and Mark Rothko? Visitors to the National Gallery may wonder as they traipse through a current show there that makes a radical departure from the gallery's traditional emphasis on recognized works from national and international sources.

The rideable rooster is actually an 1830s California barber chair for children, with a drawer in its breast feathers to hold the barber's tools. It stands ready to crow in the center of a saddled and bridled menagerie made by the Gustav A. Dentzel Carousel Company of Philadelphia and carved by Daniel Muller before 1903. The carousel animals are one of the highlights of the gallery's exhibition of folk art from the Shelburne Museum, in Shelburne, Vt., which is celebrating its 40th anniversary.

Among the 120 pieces of Americana from the Shelburne Museum are a 24-foot wooden circus broom, a 5-foot wooden tooth from an 1850 dentist's sign, and a 6-foot-tall coffee pot of sheet tin presented to J.H. Webb by his friends of the South Salem Whist Club in 1887.

The Shelburne Museum was founded in 1947 by Electra Havemeyer Webb and her husband, J. Watson Webb, as she said, ``to show the craftsmanship and ingenuity of our forefathers.'' Exhibits range from weathervanes to quilts, hooked rugs to decoys, carved figures to the sort of cigar-store Indians she liked to collect.

Mrs. Webb's test for the folk art she collected was: ``It must possess beauty, a symmetry of line or movement, an aesthetically satisfying form, or a decoration which the maker had deliberately added to please himself and the eye of the beholder.''

Many of the objects in this show fit that definition, but many of them also would look equally at home at the National Museum of American History, where such exhibits have often appeared. They include a formidable-looking fire engine weathervane, complete with prancing horses in molded copper (1890), and a mermaid weathervane of wood (c. 1825-50) in which she holds a comb and mirror for her salty locks.

Mrs. Webb's parents, Henry O. Havemeyer and Louisine Elder Havemeyer, had one of the best collections of European paintings in America. According to a gallery biography, Mrs. Webb's ``perplexed mother once asked, `How can you, Electra, you who have been brought up with Rembrandts and Manets, live with such American trash?'''

The Havemeyer family had donated many art treasures to the National Gallery. Among them: Manet's ``Gare Saint-Lazare'' and ``Masked Ball at the Opera'' and Vermeer's ``A Lady Writing,'' as well as two Goya portraits. The Webbs donated two sets of Whistler etchings and a D"urer engraving of Saint Jerome. Two Mary Cassatt works once owned by the Havemayers, ``Mother Wearing a Sunflower'' and ``Girl Arranging Her Hair,'' have also made their way to the gallery.

According to the lushly printed catalog for the exhibition, the idea for this National Gallery show came from American art scholar John Wilmerding, deputy director of the gallery. Mr. Wilmerding is also Mrs. Webb's grandson, and vice-president of the Shelburne Museum's board of trustees.

The objects for this folk art show were selected by the Shelburne staff and Wilmerding.

Among the samples of decorative art are an 1814 harvest counterpane in autumn shades, signed by Ann Robinson; an 1840 presidential wreath quilt in cherry red and green on white, done by the Traver family of Sand Lake, N.Y.; a white and blue abstract pineapple quilt (c. 1880); and an Amish Sawtooth Diamond quilt in crimson on navy, initialed J.S.B., from Lancaster County, Pa., 1880. Among the signs are a clock-shop gilded eagle holding a large watch in his beak (c. 1875) and a fish with an American flag sign (c. 1850).

The show, underwritten by a grant from The New England, will be on view at the National Gallery East Wing through April 14; then it travels to the Amon Carter Museum in Fort Worth, Texas, (May 7-Sept. 4); the Denver Art Museum (Oct. 15-Jan. 1, 1989); and later to the Los Angeles County Museum of Art; the Wadsworth Atheneum in Hartford, Conn.; the New York Historical Society; and the Worcester (Mass.) Art Museum.