With 45 exhibition halls and more than 32 million artifacts and specimens, this is the world's largest and most important museum of natural history. Dinosaur mania begins in the massive, barrel-vaulted Theodore Roosevelt Rotunda, where a 50-foot-tall skeleton of a barosaurus rears on its hind legs, protecting its fossilized baby from an enormous marauding allosaurus. Three spectacular dinosaur halls on the fourth floor -- the Hall of Saurischian Dinosaurs, the Hall of Ornithischian Dinosaurs, and the Hall of Vertebrate Origins -- use real fossils and interactive computer stations to present interpretations of how dinosaurs and pterodactyls might have behaved. In the Hall of Fossil Mammals, interactive video monitors featuring museum curators explain what caused the woolly mammoth to vanish from the Earth and why mammals don't have to lay eggs to have babies. The Hall of Biodiversity focuses on Earth's wealth of plants and animals; its main attraction is the walk-through "Dzanga-Sangha Rainforest," a life-size diorama complete with the sounds of the African tropics -- from bird calls to chain saws. In the revamped Hall of Meteorites, the 34-ton Ahnighito -- the largest meteorite on display in the world -- is accompanied by a video on space rocks narrated by Sally Ride, America's first female astronaut. The Hall of Human Biology and Evolution's wondrously detailed dioramas trace human origins and feature a computerized archaeological dig. The popular 94-foot blue whale model swims high above the Hall of Ocean Life, a "fully immersive marine environment," complete with shimmering blue lighting and whale song. For a taste of what the museum was like before computers and other high-tech wizardry were introduced, visit the softly lighted Carl Akeley Hall of African Mammals, where a small herd of elephants is frozen in time and surrounded by artful early-20th-century dioramas depicting beasts in their habitats.

The spectacular Hayden Planetarium is in a 90-foot aluminum-clad sphere that appears to float inside an enormous glass cube, which in turn is home to the Rose Center for Earth and Space. Models of planets, stars, and galaxies dangle overhead, and an elevator whisks you to the top of the sphere and the planetarium's Sky Theater, which -- using "all-dome video" -- transports you from galaxy to galaxy as if you were traveling through space. The space shows, "The Search for Life: Are We Alone?," narrated by Harrison Ford, and "Passport to the Universe," narrated by Tom Hanks, are the most technologically advanced planetarium shows in the world, incorporating up-to-the-minute scientific knowledge about the universe in computerized projections generated from a database of more than 2 billion stars. After the show, you descend a spiral walkway that tracks 13 billion years of the universe's evolution. The Rose Center also includes two major exhibits, the Hall of the Universe, in which black holes and colliding galaxies are explored, and the Hall of Planet Earth, which explains the climate, geology, and evolution of our home planet with the help of more than 100 giant rocks from the ocean floor, glaciers, and active volcanoes. On the first Friday evening of every month, 5:45-8:15, the Rose Center turns into a cocktail lounge, with tapas-style dining and live jazz under the "stars."

Films on the museum's 40-foot-high, 66-foot-wide IMAX Theater (PHONE: 212/769-5034 for show times) screen are usually about nature (climbing Mt. Everest or an underwater journey to the wreck of the Titanic) and cost $19, including museum admission. For pure musical entertainment and the latest in high-tech digital animation, SonicVision at the Hayden Planetarium is a 35-minute audiovisual roller coaster mixed by Moby and featuring music by Radiohead, U2, Cold Play and David Bowie; it plays on Friday and Saturday nights at 7:30, 8:30, 9:30, and 10:30 and costs $15. COST: Museum $12 suggested donation; museum and planetarium show combination ticket $22. Prices may vary for special exhibitions. Daily 10-5:45; Rose Center for Earth and Space stays open on Fri. until 8:45 PM. Subway: B, C to 81st St.