Scientists at Emory University have succeeded for the first time in reversing Parkinson's disease symptoms in higher animals by implanting cells from monkey fetuses into the brains of afflicted monkeys.

"We're not calling the transplant procedure a cure for Parkinson's disease," cautioned Dr. Frederick A. King, director of Emory's Yerkes Regional Primate Research Center, where the experiments were performed. "These studies are an exciting beginning," he said.

Even if future studies prove successful, it would be years before such a procedure could be contemplated for human Parkinson's victims. Beyond medical considerations, there are ethical ones: injecting foreign tissue, or human fetal tissue, in the brain could raise moral and legal questions.

No cause or cure is known for Parkinson's disease. An estimated half million Americans, most of them elderly, suffer from its symptoms of tremors, stiffness and slowness of movement. The main treatment for Parkinson's has been the drug L-Dopa, which has produced short-term improvement in about three-fourths of the people treated. The Emory experiments are among several studies seeking more effective treatment of the disease.

In the Emory study, begun in late 1984, neurosurgeon Roy A.E. Bakay harvested cells that produced the key brain chemical called dopamine, which affects movement in humans. The level of dopamine is sharply reduced in Parkinson's victims.

In the experiments, Bakay chemically mimicked Parkinson's disease in two adult monkeys by injecting the monkeys with another chemical, abbreviated MPTP. Then, dopamine-producing cells were taken from the brain stems of 35- and 37-day-old rhesus monkey fetuses to be implanted in the dopamine-deprived brains of the adult monkeys.

After two months of observation, the monkeys' abnormal movement had shown marked improvement, and the dopamine levels in their bodies had returned to normal.

The animals did not fully resume their earlier level of activity. Nonetheless, the work "represents a promise of future transplantation in Parkinson's disease . . . ," Bakay said in a paper prepared for delivery at the World Congress of Neurosurgery in Toronto next week.

Bakay's results sound "terrific . . . very encouraging," said Dr. William J. Freed of the National Institutes of Mental Health in Washington after hearing a description of the experiment. "No one has accomplished that before."

Like Freed, Dr. Richard J. Wyatt of NIMH said he looked forward to "seeing their data in scientific journals. I hope they have continued success when they can observe and follow the animals for a longer time."

Dr. Murray Goldstein, head of the National Institute of Neurological and Communicative Disorders and Stroke, one of the National Institutes of Health, said the Emory research was a "logical next step" among scientists who have been studying brain implants as potential treatment for Parkinson's.

Implants previously have reversed the onslaught of Parkinson's in rats. Brain tissue has been successfully implanted in monkeys before but dopamine-producing cells did not take well and the Parkinson's symptoms were not substantially reversed.

Bakay said in an interview that harvesting and implanting the fetus cells at precisely the right age of development appears to be a key to success. Studies elsewhere have shown that in cell development, there is a "narrow window" of 35 to 41 days in which fetal cells may be transplanted most effectively. At that point, the cell is ripe for linking with tissue into which it is thrust, he said.

Other researchers trying to reverse Parkinson's with fetal implants have used older tissue, he said.

Within the last year, Swedish doctors have implanted adrenal cells into the brains of two human patients. Improvement was short-lived. The surgery has been widely criticized by American doctors as premature.

Bakay said he plans to apply this fall for an NIH grant to compare treatments with the fetal tissue, adrenal tissue, and conventional drugs used with Parkinson's.

Even if his line of study succeeds, Bakay said he realizes Americans may reject foreign-tissue implants in the brain on ethical grounds. "It may turn out that the fetus is the best thing to do but that society will only let us do adrenal implants. If so, we'll do adrenals. But all of that is years down the line."