Biologists have found a natural process that makes female animals -- including humans, apparently -- less sensitive to pain during childbirth.

Some groups have maintained for years that childbirth can occur with relatively little pain despite the long physicial trauma involved. These groups have also noted that peaks of pleasure are experienced by some women who give birth without chemical or surgical aid.

The latest research to lend credence to the position, to be published today in Science magazine, is a report by Dr. Alan Gintzler of the Downstate Medical center in New York.

Gintzler speculated that his work and others' eventually may give enough understanding of the body's method of pain relief to use that natural system to ease pain in childbirth.

"This might help eliminate the use of pain-killing drugs which have been found to be harmful to infants in many animal studies, but which are still widely used in hospitals," according to Dr. Candace Pert of the National Institute of Mental Health, one of the discoverers of the body's system for relief of pain.

The key to this system is a family of brain chemicals called endorphins, which have profound effects on sensations of pain or pleasure. Scientists have found that they connect with special receptors in the brain to block pain. h

Gintzler, in the study reported in Science, tested the pain tolerance of pregnant rats from 16 days before birth until 41 days after. He placed the rats on a charged grid to determine what level of electric shock to the paws the rats would tolerate before jumping off.

The result was a graph shaped like a mountain peak -- the rats tolerated pain about normally 16 days before giving birth. But as the date approached their tolerance grew, until, near the time of birth, it had nearly doubled. The pain tolerance then subsided gradually over the next two weeks to a near-normal level.

Gintzler said he believes the endorphin system is responsible for this effect.

Studies of the same effect in human mothers are now being done by Hude Akil and Cheryl Cahill at the University of Michigan. Akil said preliminary studies on herself and a small number of other women showed that in the pregnant women tested, endorphins were found at six or seven times the normal level. The amount seemed also to jump even higher during birth, she said.

Until the larger study is done, however, the effect cannot be certain, especially the degree to which pain or pain relief is linked to the amount of endorphins in the blood.

Gintzler, who contducted the rat studies, said the endrophin systems "might be an intrinsic mechanism that helps the animal sustain itself when confronted by pain."

Gintzler said he believes that what caused the great tolerance to pain was the increasing presence of endorphin molecules in the rats' systems, or some other activaiton of the endorphin system. The endorphin molecules, naturally produced by the brain in animals and man, act very like opiates -- heroin or opium -- except that they are far more powerful in pure form.

He tested the rats to see if the endorphin system was the active agent by administering a chemical called naltrexone, which is known to block the action of endorphins and oplates in the body. The pregnant rats given naltrexone did not experience extra tolerance of pain, thus strongly suggesting that the endorphin system is the agent of relief.

Other researchers have tested rats' blood and found that the level of endorphins increases several times during pregnancy.

The question that remains is what triggers the endorphin system during pregnancy, and whether it is directly a response to pain and stress.

In some preliminary research by Pert at the NIMH, it appeared that one factor in the amount of endorphins present in the blood was the condition under which the rats delivered. In crowded, nosy conditions, less endorphin was recorded. A quiet atmosphere appeared to increase the level of action of the endorphin system.

Though this work was inconclusive, Pert said, it suggests that the more natural the setting for birth, the more endorphin which may be active.