For a week in 1996, Greg M. was one of Dr. Ricaurte's lab subjects.

At the time, he said, he was using large amounts of Ecstasy, marijuana, LSD, cocaine, amphetamines and heroin.

After seeing the retraction of the primate study, he contacted The Times, and persuaded a friend who had accompanied him to call, too.

The two revealed their names and occupations but declined to be fully identified for fear their former drug use would hurt their careers. Greg is a graduate student in chemistry at a leading university. His friend, who said he used to follow Grateful Dead tours selling up to 10,000 doses of LSD a month, now works at a West Coast law firm and is in line for a federal job.

Curious to see if they had damaged their brains, and enticed by a promise of $100 a day and a free East Coast trip, they enlisted.

Although the two used many drugs, the research assistant who interviewed them by phone told them what not to admit to her if they wanted to be in the study, Greg said. They were instructed to avoid all drugs for three weeks to avoid tainting the study; Greg says he had used heroin five days earlier.

They and other Ecstasy users flown in from the West Coast took memory tests while still jet-lagged, they said.

Then after lumbar punctures to check serotonin levels, neither was given the usual night's rest to prevent fierce headaches. They had to carry their backpacks across campus and be wired up for a sleep study, which Greg argued could not reflect normal sleep patterns because they were in pain.

Both had subsequent tests after shots of morphine and a drug, mCPP, that causes the same eyeball twitching and teeth-grinding as Ecstasy, but none of the euphoria. Then they had PET scans.

Dr. Ricaurte said his research protocols are approved by university committees. He acknowledged testing sedated or jet-lagged subjects, but argued that he had always noted that limitation in his published papers, and switched to testing in early mornings when jet lag was minimal. Test subjects who get lumbar punctures are warned about headaches, and given rest and painkillers, he said.

To weed out subjects who confound results by using other drugs, Dr. Ricaurte said, his staff quizzed volunteers and did blood and urine tests. His papers acknowledge that hair tests, which can show many drugs taken even months back, would have been more accurate. (Dr. Laruelle, who does PET scans of Ecstasy users, rejects subjects with hair less than an inch long.)

Told that Greg had used heroin without getting caught, Dr. Ricaurte said that was "unfortunate." But like all drug researchers, he said it was impossible to find heavy Ecstasy users who used no other drugs.

His papers, he said, always warn that poor performance by heavy Ecstasy users may have been caused by other drugs.

His critics say that such fine-print disclaimers are not enough, that all mental tests on multiple-drug users are pointless and cannot be used as evidence that one particular drug damages the brain.

Greg's friend reiterated that he had been badly treated and said he felt the research was skewed to prove he was brain-damaged.

"Most of the people I used to do drugs with are pretty screwed up," he admitted. "But if Ricaurte's studies are true, Greg and I should both be dead. We ate grams a night of pharma-grade stuff."

Nonetheless, he said: "We're fairly intelligent, rational guys. We had a stretch of three or four years where we really blew ourselves out. But we're still smart and ambitious. Some of their assertions about long-term brain damage are way off."