For Alex Pacheco, co-founder and chairman of People for the Ethical Treatment of Animals (PETA), a single horrifying event provided the impetus to become an activist.

"Once, on a college break, I visited a slaughterhouse where my friend was working. Among other horrors, I saw workers slit a live sow's belly open, pull out her unborn piglets and play catch with them. It turned my stomach and turned me vegetarian," wrote Pacheco in a PETA newsletter.

After that incident, the theological student abandoned dreams of the priesthood and has since pursued the protection of animals with religious devotion.

He launched his career as an activist with the Sea Shepherd Conservation Society, a non-governmental watchdog group. Its members ride the ocean on a renovated fishing boat called the Sea Shepherd, tracking whale, seal and fishing operations to monitor their compliance with international and domestic laws.

Pacheco was a member of the Sea Shepherd crew during its encounter with the Sierra, an outlaw whaling ship that may have killed up to 25,000 whales between 1968 and 1979, according to Society estimates. In 1979, the Sea Shepherd caught up with the Sierra off of the coast of Portugal. With its concrete-reinforced bow, the Sea Shepherd charged the Sierra, colliding with it three times. Pacheco was later named "Crew Member of the Year" for his efforts during that expedition.

His breakthrough occurred two years later with the first successful prosecution in the United States of a scientist on charges of cruelty to animals. Posing as an assistant in the Silver Springs laboratory of Maryland researcher Edward Taub, Pacheco documented the substandard treatment of several monkeys.

His report painted a picture of rodents and cockroaches running freely through the laboratory as sick monkeys languished in their cages. Taub had paralyzed one of the arms of half of the monkeys in order to test their responses to cigarette lighter burns and other stimuli.

His role in Taub's conviction launched Pacheco into a high-profile position that benefited PETA, the then-fledgling organization that he launched in 1980 with activist Ingrid Newkirk.

Nineteen years later, PETA has grown into the largest animal rights organization in the United States, with more than half a million members. Pacheco is credited with successfully courting celebrities to join the fight against PETA's targets: factory farms, laboratories, the fur trade and the entertainment industry. Indeed, it was largely through PETA's efforts that fur popularity declined in the early 1990s.

PETA's critics range from mocking to deadly serious. There is the web site entitled "People Eating Tasty Animals". There are also the threats against Pacheco's life.

Even an article in the liberal magazine Utne Reader criticizes PETA's stance on fur. "It's not hard to see that the attacks on fur-wearing females (as opposed to leather-wearing men) play simultaneously on cheap populism and cheaper sexism. You can scream at women in mink coats emerging from ritzy department stores and be fairly certain they're not going to physically retaliate...It would be more interesting to watch zoophiles gathered in front of a biker bar, hollering slogans at the leather-sporting clientele as they swagger up to their Harleys. But we're not likely to see that anytime soon, are we?" wrote Richard Ryan.

Through all of the criticism, Pacheco has remained constant in his dedication to promoting the humane treatment of animals. "Every action you take, every letter you write, every time you speak up for animals, it gives them another chance to live in peace," wrote Pacheco in a PETA newsletter. "Please never miss an opportunity to make a difference."