An afternoon at the zoo, wherever you travel, can provide an agreeable break from city sightseeing. In the United States there are about 240 public zoos, from major scientific institutions such as Washington's National Zoo -- whose pandas Ling-Ling and Hsing-Hsing help draw more than 3 million visitors a year -- to small parks featuring regional wildlife. Among the most interesting and innovative:

* Arizona-Sonora Desert Museum, Tucson: One of America's finest regional zoos, it exhibits the creatures of the southwestern desert, from bighorn sheep to prairie dogs and tarantulas.

* Audubon Zoological Gardens, New Orleans: With the Mississippi River in the background, you stroll a raised walkway above a recreated Louisiana swamp, peering into the murky habitat below of alligators, raccoons, opossum and otter.

* Point Defiance Zoo & Aquarium, Tacoma, Wash.: This is a fine representation of wildlife of the Pacific Northwest, highlighted by an impressive polar bear exhibit. You can watch them dive and swim beneath the water.

* Milwaukee County Zoo: The Great Ape Escape, an outdoor gorilla exhibit, draws the crowds, and so do the penguins, especially the appealing little "rock-hoppers."

* Metrozoo, Miami: A large, new cageless zoo, where the animals reside in natural settings surrounded by moats. The key attraction in the Asian section is white Bengal tigers, prowling beneath a temple from ancient Cambodia.

* Philadelphia Zoological Garden, Philadelphia: America's first zoo (founded in 1859) and still among the best. Inside the Jungle Bird Walk are five species of hummingbirds.

* Washington Park Zoo, Portland, Ore.: The Cascade Stream and Pond habitat is home to beavers, otters and other creatures of the Cascade Mountains, but the zoo is also famous for its herd of Asian elephants.

* Minnesota Zoological Gardens, Minneapolis/St.Paul: A pair of Beluga whales splashes year-round in their 495,000-gallon outdoor pool. You can watch them being hand-fed 70 pounds of fresh fish daily.

* Gladys Porter Zoo, Brownsville, Tex.: A small, charming zoo in a beautifully landscaped Gulf Coast garden.

* Cincinnati Zoo, Cincinnati: The Insect World is the largest exhibit of its kind in North America. You can watch a colony of 200,000 leaf-cutting ants from South America or step into the walk-through world of gorgeous tropical butterflies.