The discovery four years ago that the brain contains cells to which morphine attaches itself prompted a quest for the roots of pleasure, pain and mental disorder - and a related search for the body's own source of the drug.

Using radioactive substances, scientists found that only certain cells in the brain would accept these substances and that, in the process, pain responses are blocked. The scientists also found that the substances were accepted in the amygdala, a part of the brain believed to be asociated with emotional functions. Eventually they found the substance: enkephaline.

In the course of this reseach, it was noted that enkephalines bear a remarkable resemblance - a sequence of five amino acids - to a substance discovered in 1965 by Dr. C.H. Li of the University of California, San Francisco. Li found this substance in the anterior pituitary while studying growth hormones and labeled it betalipotrophine. He had no idea what the substance did.

The morphine-receptor researchers postulated that enkephaline might come from Li's mysterious substance. Li responded by suggesting that the answer might be found it an animal with tremendous tolerance for pain was studied.

Li's hunch: the camel. You have to beat it with a baseball bat to get it to move, and the animal will literally consume itself without eating or drinking. He dispatched a graduate student to the Middle East for several years to study the pituitary glands of 1,000 camels. The finding: One part of the animal's gland contained more of the amino acid chain (specifically molecules 61 through 91) than any other animal. Dr. Eric Simon in New York named it beta-endorphine: endrophine for the body's own morphine; beta for the portion of the chain it occupied.

Li began sythesizing the drug in the lab. By June of this year he had created 100 milligrams of the white liquied, which was provided to Dr. Nathan Kline for use on patients with severe psychiatric disorders.

"There's no question the medication is working," says Kline, "for as long as a week at a time. The immediate problem is that a 10-milligram injection costs $3,000."