While levels of beta-endorphin decreased in response to licking, the levels rose when the infants were taken from their mothers. If the separation persisted, the infant rats' growth was stunted.

But resumption of the mother's touch, even when simulated with a brush, again lowered the beta-endorphin levels and quickened growth.

''We believe that the brain effects we found in rats will also hold for humans, because the basic neural and touch systems are the same,'' Dr. Schanberg said. Primitive Survival Mechanism

He hypothesizes that the touch system is part of a primitive survival mechanism found in all mammals. Because mammals depend on maternal care for survival in their early weeks or months, the prolonged absence of a mother's touch - more than 45 minutes in a rat, for instance - triggers a slowing of the infant's metabolism, and thus a lowering of its need for nourishment. Such a reaction heightens its chances of surviving until it is once again in contact with the mother.

While the slower metabolism is beneficial in the short term, it stunts growth if very prolonged. According to Dr. Schanberg, part of the response in rats, which includes huddling down and becoming still, is a change in metabolism that conserves the store of energy and slows the rate of growth. The mother's touch, however, reverses the process, so that growth resumes at normal rates.

In related findings, physical contact with the mother appears essential to reducing the release of hormones by an infant when subjected to stress, according to Seymour Levine, a psychologist in the department of psychiatry at Stanford University Medical School. When infant rats or monkeys are separated from their mothers, activity in the pituitary-adrenal system rises, a response that is also typical for humans under stress. In Dr. Levine's studies, physical contact with the infant's mother lowered this stress response. Beyond Mere Proximity

Contact and touch have a significant role in the infant's ability ''to regulate its own responses to stress,'' Dr. Levine said. His work does not allow him to separate touch in itself from the more general effect of the mother's presence, but he theorizes that in humans a touch-induced reduction of stress hormones may account for the soothing effects of skin-to-skin contact.