A Catholic University nursing student died Saturday while participating in a National Institute of Mental Health experimental program designed to monitor how two drugs -- one of them not yet on the market -- affect mood and sleep patterns.

Institute officials said the paid volunteer, Bernadette Gillcrist, 23, a senior nursing student from Bethesda, died in her sleep at the Rockville research complex at approximately 5 a.m. She was one of 20 volunteers being used to test the combination of lithium, an antidepressant, and AMPT (alphamethyl-para-tyrosine), a relatively new drug designed to decrease the body's flow of adrenaline.

Dr. Mortimer Lipsett, director of the National Institutes of Health clinical center that oversees volunteer programs at NIMH, said earlier this week that Gillcrist had died of cardiac arrest and that she had a history of heart disease that she had concealed from NIMH officials.

Yesterday, however, the state medical examiner's office disputed that theory, saying an autopsy performed on Gillcrist failed to reveal the cause of death. The result is pending while further toxicology tests are performed.

"There's no telling what she died from," a spokesman for the office said.

According to lipsett, Gillcrist volunteered for the study in January and was to receive $800 for participating in the experiment.

Dr. Chris Gillin, director of the sleep study, said yesterday that Gillcrist had been given a complete medical examination before the study, including blood tests, chest X-ray and electrocardiogram. Asked if her death could be linked to the drug experiment, Gillin said, "We don't think so. We really don't think so. But you just don't know. We don't understand the whole thing yet, frankly."

According to Lipsett, Gillcrist had suffered a heart attack several years ago and had been a patient of the National Institutes of Health, unbeknown to the clinical center.

"The girl was sick," Lipsett said yesterday. "She obviously shouldn't have been a volunteer. I don't know why someone would lie to us. I just don't know how it was disguised."

Gillcrist's death marks the first time an outpatient volunteer has died at NIH while participating in an experimental study. "It's bizarre," Lipsett said yesterday. "It's never happened before."

Gillcrist's older sister had also volunteered for the sleep program, Gillin said yesterday, but was turned down for medical reasons. To participate in the paid experimental programs, volunteers must have no history of mental or physical problems.

The Gillcrist family declined to be interviewed yesterday. "I'd rather not say anything," Gillcrist's mother, Marcia, said from her Bethesda home. "I'm really in an upset frame of mind. The autopsy had been done. The results are pending. All this has happened so fast."

According to Lipsett, Gillcrist -- and 19 other volunteers -- had been given small doses of lithium, recognized as an effective drug for treating manic depressives, for seven days before her death.

Gillcrist spent the night at the clinic for the first time on Wednesday and her sleep and mood patterns were monitored by an encephalogram. She returned the following night, for a second night of sleep.

On Friday, according to Lipsett, Gillcrist was given three doses of AMPT, in the morning, the afternoon and again in the early evening. She left the hospital around 8 p.m., Lipsett said, to visit her private physician. lAccording to Lipsett, Gillcrist did not tell her private doctor she was participating in the NIH study.

By 10 p.m., she had returned to the clinic and gone to sleep in a small room with a clinic technician "10 feet away," Lipsett said yesterday.

Around 5 a.m., the doctor said, the technician monitoring Gillcrist noticed something wrong on the encephalogram. "Everything went flat," Lipsett said. Rather than try to arouse the sleeping volunteer, the doctor said yesterday, the technician assumed that the machine was "out of whack" and did not summon assistance.

Two hours later, the technician discovered that Bernadette Gillcrist was dead.

"Whether she could have been saved, I honestly don't know." Lipsett said yesterday. "The girl died suddenly."

Friends of Gillcrist said yesterday they had no idea she had health problems.

"As far as we knew, she had no cardiac defects at all," said Lynn LaCaruba, who lived across from Gillcrist on the first floor of Regan Hall at Catholic University. "She ran, didn't smoke, was a bright, serious student who worked hard and never went to parties. I don't even think I ever saw her with a drink in her hand."

A spokesman for the university's nursing school said Gillcrist's student medical record showed no past heart ailment.

In addition to being a paid volunteer, Gillcrist also worked part time at the National Institute of Cancer.

"What I'd like to know," said LaCaruba, "is why someone wasn't monitoring her more closely. You'd think they'd be able to tell something's not right."

Lipsett said the sleep laboratory has been operating for 25 years and that the study on the effects of Lithium and AMPT had already been performed on 30 other volunteers. Gillin, the study's director, also noted that the two drugs had been tested before without any problem at other research centers.

Bernadette Gillcrist is scheduled to be buried today. "It's a real tragedy," Lipsett said.