Strong new evidence has emerged that mammals, probably including humans, naturally produce morphine and codeine in their tissues. The substances are chemically identical to the pain-killing narcotics, which are extracted from the opium poppy.

It has been known for years that the body makes opium-like chemicals, called endorphins and enkephalins, that have effects similar to those of morphine. These molecules, however, are small proteins called peptides and, although the effects are similar, are completely different chemically from the morphine and codeine produced in the opium poppy.

Heroin is a chemically modified form of morphine that passes more quickly from the bloodstream into the brain. Once in the brain, it is naturally reconverted to morphine, the form in which the substance acts on the brain.

Researchers have long suspected that the body may make its own morphine because brain cells have receptors on their surfaces shaped specifically to fit morphine's shape. It is this lock-and-key fit that causes the receptor to bring morphine molecules into brain cells, a necessary step before morphine can work. It made no sense to biologists that the brain would have receptors for a substance produced only by poppies.

Over the last decade, laboratories have detected morphine and codeine in the skin of toads, rats and rabbits and in the brains of cattle. Morphine has even been found in cow's milk. But in every case the quantity was so small that contamination from external sources could not be ruled out.

Approaching the question from another direction, a team of biochemists at Stanford University looked for evidence that mammalian cells could carry out the biochemical reactions necessary to synthesize morphine. If mammals make morphine, they reasoned, they may do it the same way the poppy does. So the researchers looked for molecules that serve as morphine precursors, intermediate substances in the poppy's sequence of steps toward a morphine molecule.

They succeeded. Charles J. Weitz, Kym F. Faull and Avram Goldstein, a pioneer in this field, reported in Nature magazine that they have found in rat livers the highly unusual ringlike molecules that are the "skeleton" of a complete morphine molecule.

The finding strongly indicates that the earlier detections of morphine in mammalian tissues were real. If the work is confirmed, it may eventually complicate the work of crime labs looking for evidence of illegal narcotics in human tissue.