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Arizona's Winkelman said he believes there is something in human biology that makes us want to reach for such altered states. And one infamous incident known as the Good Friday Experiment seemed to show you didn't have to practice shamanism to have a spiritual experience with hallucinogens.

On Good Friday 1962, some researchers at Harvard gave a small group of divinity students either psilocybin or a placebo. Psilocybin, then legal, works much like peyote. "Eight of the nine people who got the drug reported they had had the most profound spiritual experiences of their lives," Winkelman said.

People use the term hallucinogen loosely to apply to many mind-altering drugs, but peyote belongs to a small family that share similar modes of action on the brain. They include psilocybin, LSD, and morning glory seeds.

The chemical structure of these resembles a critical messenger molecule in the brain known as serotonin, said David Nichols, professor of medicinal chemistry and pharmacology at Purdue University. When serotonin is created in the brain it works by attaching, lock-and-key fashion, to molecules called serotonin receptors.

The brain has 14 different types of serotonin receptors, said Nichols, and the hallucinogenic substances dock in just one of these, called the 5HT2a receptor. "You get the overstimulation of one receptor at the expense of the others," he said.

Hallucinogens act on receptors in the frontal cortex, sometimes called the executive part of the brain because it's used for higher reasoning, he said. They also act on a part of the brain called the thalamus, Nichols said, which works to help us distinguish what's novel and important. That may explain why people on LSD can become mesmerized by a flower or by their own hand.

Studies like the Good Friday Experiment ended after psilocybin and other hallucinogens were made illegal in the late 1960s and early '70s, but a handful of scientists today are looking at ways these types of drugs might help people.

John Halpern, associate director of substance abuse research at Harvard University and McLean Hospital, is investigating the possibility that peyote prevents alcoholism in American Indians.

In a study he plans to publish within the next several months, he compared cognitive and psychological health measures among Indians who were alcoholics, those who regularly used peyote, and those who used no drugs or alcohol.

Halpern said he can't reveal his results yet, but he will say he sees no evidence that peyote damages the brain. "There's no history of it being addictive, or trafficked or abused," he said. Peyote can be dangerous if people use it to get stoned and then do stupid things, he said, but that's not what happens in religious ceremonies

"I've never seen any harm coming from this. In fact it's just the opposite - it really brings families together," he said.

Others, such as David Murray of the Office of National Drug Control Policy in Washington, see more serious risk. Working among the Navajo, he said, he found long-term peyote use was "counterproductive to education and social mobility."

Because the peyote comes from a natural plant, he said, "you're taking in a powerful chemical stew," with some toxins in addition to the psychoactive ingredient. "It is, without question, a risky undertaking."