THE DEPARTMENT of Commerce looks like any other innocuous federal office building with huge columns and heavy brass doors punctuating its classical facade. But deep in the basement is a veritable lagoon: Alligators swim in clear pools, sharks dodge through a forest of sea kelp and piranhas cluster in schools, waiting, oh so patiently.

Unbeknownst to most Washingtonians, the Commerce Department once developed fish-breeding programs and related technologies to stimulate the economy. In order to conduct research and showcase its efforts, the department established the National Aquarium in 1873; its 14th Street headquarters became the aquarium's permanent home in 1932. Though it lost federal funding during the Reagan administration and became a private nonprofit, the aquarium -- the nation's oldest -- remains in the Commerce building.

Its relatively obscure location, coupled with the proximity of the high-profile National Aquarium in Baltimore, might be viewed as a publicist's nightmare. But Executive Director Robert Ramin doesn't view such things as problems. "We see ourselves as complementary, not competing," he said. "We have some things they don't have," such as the venomous lionfish, the endangered bonytail chub and the striped chambered nautilus. Indeed, Ramin says he thinks the smaller size of the Washington aquarium is an advantage to visitors. "It's not an overwhelming place to go; a family can see it all in 45 minutes. And it's a more intimate experience. You can really see the species."

In keeping with the aquarium's "up close and personal" sensibility, the staff offers daily lectures and feedings of bloodthirsty favorites -- piranhas, sharks and alligators. The fish are fed three times a week, and the alligators are fed only on Fridays. "These are all coldblooded [animals] with slow metabolisms," explained Harry Chow, one of the aquarium's curators. "When they have a meal, they're good for a couple of days. They don't need as much energy from food as we mammals do."

On a recent Tuesday, I joined the piranhas for lunch. Despite their ferocious reputation, they look pretty benign, with gray scales and a peachy underbelly flecked here and there with gold. Each has a small, smooth mouth with a slightly protruding underjaw.

Chow, also the piranha caretaker, warns onlookers not to do anything to agitate the fish. "They don't like having a lot of people crowding around the tank, tapping on the window," he said. They don't like flash cameras, either. "It will make them nervous to the point that they won't eat. Once a kid used a laser pointer on the fish -- they couldn't eat for days." (Maybe they should have thrown the perpetrator in the tank.)

Generally the piranhas are quite still, most facing the same direction as if in military formation. At feeding time, however, all hell breaks loose. Chow tossed in a long piece of ocean perch. One piranha quickly snatched it up and swam around the tank, the other fish trailing in hot pursuit.

Occasionally the fish nip at one another -- or worse. "They eat the sick and the dead," Chow said. "Every five years we have to replace them." Once, he said, it got to the point where there were only two left in the tank. "They are quick healers, though," Chow said, pointing out a fish with a tear on its dorsal fin.

The next day, I returned to see the sharks. In marked contrast with the greedy piranhas, they had an almost indifferent attitude toward their meals. "Lots of people come here with images of man-eating sharks," says aquarium curator Jay Bradley. "But we want to give you a different idea about what sharks look like." The aquarium exhibits four species -- horn, swell, leopard and cat sharks -- all bottom dwellers. Most are soft brown or gray with polka-dot markings, and they lead a fairly modest existence, scavenging for food on the ocean floor. Their teeth are short and flat, ideal for crushing such tasty invertebrates as clams and sea urchins.

The day I visited, shrimp was on the menu, already shelled, cooked and, I suspected, deveined. "We avoid giving the sharks live food; [it] might introduce disease," Bradley said. He tossed a couple of handfuls into the tank, and they drifted to the pebbled bottom. Occasionally a horn shark would deign to scoop one up. I had seen more aggressive feeding behavior at wedding receptions.

D.C. resident Alicia Coulter, 9, speculated on another shark's lack of motivation. "He's just sitting there opening and closing his mouth. He's tired maybe."

"They don't seem that smart; I feel safer now," said Jamie Wilkinson, a visitor from Utah. Her son Blake, 10, however, was more impressed. "They're awesome; they're the real thing," he said.

Bradley explained that the sharks' relaxed approach to meals was not unusual. "These species are particularly mellow. They aren't going to chase a fish." In addition to the sharks, Bradley cares for the mollusks and bivalves at the aquarium, "which are pretty cool, but," he admitted, gesturing toward the shark tank, "these guys are more interesting."

I took Thursday off and stopped by Friday for the alligator feeding. "I've always loved reptiles," said Melanie Litton, the aquarium's herpetologist. "I'm fascinated by how they have survived and evolved."

The alligators are a handsome pair, with rough, black and white striped skin and smooth underbellies. Their eyes are high atop their heads and have vertical pupils -- an adaptive feature that enables them to watch for prey on the water's surface while their bodies are submerged. Their wide jaws have a bit of an overbite, revealing rows of small, yet exquisitely sharp, teeth.

The alligators were resting under a sunlamp when they were spied by two young visitors, Gabriel Guny, 4, of Palm Beach, Fla., and Mason Lomeli, 21/2, of Sacramento. "He's got big eyes," Mason said. "If I fall down, he might eat me," Gabriel speculated.

When Litton arrived with their lunch, the alligators became more animated, diving into their exhibition pool and swimming around. "They know they're about to be fed," said Litton, who had a dead rat and quail for each. Before distributing the goodies, however, she warned visitors to watch carefully.

"If you blink, you miss it."

As she opened the exhibit door, the alligators crowded around, snapping at her feet. Using a long-handled pole with a pincher, Litton picked up a rat and extended it to an alligator that promptly snapped it up, raising its head and shaking it violently from side to side as if trying to swallow a big vitamin. The second alligator held a rat in its mouth with only the tail hanging out. "It looks like a toothpick," snickered Dominic Muzzin, 10, of the District. Suddenly, the first alligator made a grab for the rat tail, snapping it off. Visitors shrieked in unison.

Though the visceral pleasure of watching these coldblooded creatures tuck into their lunch is undeniable, Litton said she wants her alligators to do more than entertain. "What I'm hoping is that people will better understand the animal and its actual behavior and forget the myths," she said. Her goal is shared by the other caretakers. Though horror movies about piranhas, sharks and alligators titillate us, lunch hour at the aquarium proves that the animals' real-life feeding behaviors are far more fascinating.

NATIONAL AQUARIUM IN WASHINGTON -- 14th Street and Constitution Avenue NW. 202-482-2825. www.nationalaquarium.com. Open daily 9 to 5. Keeper talks and animal feedings daily at 2. Sharks are fed Mondays, Wednesdays and Saturdays; piranhas Tuesdays, Thursdays and Sundays; and alligators Fridays. $5; seniors $4; children 2-10 $2; children 1 and younger free. Credit cards are not accepted.

An alligator quickly snatches a rat out of the air during a public feeding at the National Aquarium in Washington. Alligators at the aquarium are fed just once a week. Cory Sullivan, 5, of Cary, N.C., sees eye to eye with an alligator inside the Commerce Department building.