This museum grew out of a gallery in the studio of the sculptor and collector Gertrude Vanderbilt Whitney, whose talent and taste were fortuitously accompanied by the wealth of two prominent families. She offered her collection of 20th-century American art to the Met, but they turned it down, so she established an independent museum in 1930. The current building, opened in 1966, is a minimalist gray-granite vault separated from Madison Avenue by a dry moat; it was designed by Marcel Breuer, a member of the Bauhaus school. The monolithic exterior is much more forbidding than the interior, where exhibitions offer an intelligent survey of 20th-century American works. The fifth floor's eight sleek galleries house "Hopper to Mid-Century," with works by Reginald Marsh, George Bellows, Robert Henri, and Marsden Hartley. Notable pieces include Hopper's Early Sunday Morning (1930), Bellows' Dempsey and Firpo (1924), and several of Georgia O'Keeffe's dazzling flower paintings. Postwar and contemporary highlights from the permanent collection include paintings and sculpture by such artists as Jackson Pollack, Jim Dine, Jasper Johns, Mark Rothko, Frank Stella, Chuck Close, Cindy Sherman, and Roy Lichtenstein. The museum lobby is home to Alexander Calder's beloved sculpture Circus (1926-31) with tiny wire performers swinging from trapezes and walking on tightropes. The famed Whitney Biennial, which showcases the most important developments in American art over the past two years, takes place in even-numbered years. The Whitney also has a branch across from Grand Central Terminal in midtown. COST: $12; Fri. 6-9 pay what you wish. Subway: 6 to 77th St.