Murals are the grandest and most effusive of art forms -- and the oldest, if the evocative beauty of the cave painting is any indication. A good mural can tell a story, evoke an era, or embolden a landscape with panoramic brushstrokes. San Francisco is blessed with several fine examples of this boundless medium; here are three of them.

San Francisco Art Institute

800 Chestnut (at Jones), 771-7021

Diego Rivera's 1931 fresco is a fresco about a fresco -- it depicts a group of muralists (Rivera among them) perching on a scaffold, plying their trade. Their subjects are those heroes of so many Depression-era murals, organized labor, shown here drilling rivets, placing beams, pulling throttles, and in general making America run, while three clean-fingernailed capitalists stand idly by. Also in this self-reflective classic is Rivera's artist wife, Frida Kahlo, pulling a cameo in the lower right corner as she toils away at a drafting table. The mural is displayed in a tall, light-filled space along with revolving exhibitions of student artwork.

Rincon Center

101 Spear (at Mission), 243-0473

During the Depression the Treasury Department's Section of Fine Arts commissioned artists to paint murals in post offices across the nation to rouse the locals with stirring scenes of local significance. When the Rincon Center Annex Post Office was completed in 1941, Moscow-born artist Anton Refrigier was hired to fresco the walls with a history of Northern California. The result was damned as "little short of treason" by the San Francisco Young Democrats League, underwent 92 changes, and was still ostensibly radical enough to be threatened with removal by state Assemblyman Hubert Scudder in 1953. Today this 29-cycle epic of social realism is unlikely to inspire much more than artistic admiration, if only because the Rabelaisian friars, the books with the inflammatory titles, and other offenses were so carefully whitewashed away half a century ago. Still and all it's a marvelous, sweeping historical saga.

Balmy Alley

24th Street between Treat and Harrison

Back in 1973, local children, artists, and community activists started painting the garage doors, buildings, and flowering fences along this (undeniably balmy) alleyway to promote peace in Central America. Today it's one long, beautiful block of vibrant street art celebrating childbirth, folklore, the UFW, community feeling, and resistance to tyranny north and south of the border. Brilliant shades of purple, yellow, orange, green, and blue are employed in a myriad of styles ranging from narrative to abstract to good old-fashioned social commentary.