Three-time heavyweight boxing champion Muhammad Ali has decided not to have experimental brain surgery to help relieve the symptoms of the Parkinson's disease-like syndrome that has slurred his speech and slowed his movements for the past several years.

Ali, who was examined by Dr. Ignacio Madrazo of the Universidad Nacional Autonoma de Mexico in Mexico City, returned to the United States yesterday. He reportedly has made plans to be reexamined by American experts in Los Angeles and New York City.

Ali, 45, had been in Mexico to attend the World Boxing Council's medical symposium in Cocoyoc. It is not clear whether Madrazo, a member of the Mexican research team pioneering a controversial procedure for Parkinson's patients, initially approached Ali to offer him the surgery or whether friends of Ali encouraged him to seek out Madrazo to learn about the operation.

Business associate Larry Kolb, who met Ali at Chicago's O'Hare airport yesterday, said Ali said that Madrazo approached him about the procedure.

Madrazo, who has declined to talk to the media about Ali, reportedly thought the surgery might be helpful, although he agreed that Ali's symptoms were far milder than problems faced by his previous patients.

Ali, who was traveling yesterday, could not be reached for comment.

"I had advised him not to have it done," said Ali's primary physician, Dr. Dennis Cope of the University of California at Los Angeles Medical Center. "I think Ali's condition is not severe enough to warrant the risk of the procedure."

The experimental surgery, performed on 18 Mexican patients in the past 1 1/2 years -- and about 50 patients world-wide -- attempts to limit the symptoms of Parkinson's disease by transplanting cells from one of the adrenal glands -- there are two, one on top of each kidney -- to an area of the brain that controls movement.

Parkinson's arises when cells that produce dopamine -- a brain chemical essential for voluntary movement -- are lost with age. For most Parkinson's patients, the disease continues to progress, and the drugs used to control the symptoms eventually become useless.

The surgery is believed to help because the transplanted adrenal cells are capable of producing the missing dopamine and making it available to the cells that need it.

In Ali's case, "There is no evidence that it is going to progress and lead to invalidism," said Dr. Stanley Fahn, a neurologist at Columbia University College of Physicians and Surgeons who has examined Ali in the past and talked with his representatives yesterday to set up an appointment.

Although results from the Mexican study and others, presented two weeks ago at an international meeting in Rochester, N.Y., were dramatic, they were very controversial. Reseachers at the meeting were not convinced that the transplanted cells were really responsible for the dramatic effect seen in videotapes of some of the patients.

Neurosurgeons around the United States, however, have begun transplanting adrenal cells in the brains of a number of Parkinson's patients to try to duplicate the Mexican results.

In Ali's case, the surgery would have been even more complicated, and, perhaps, more controversial.

Over the years, the pounding Ali took in the ring caused damage to areas of his brain that control voluntary movement. "He has a form of Parkinsonian symptoms that have been the result of pugilistic brain syndrome," Cope said. Such damage results from frequent blows to the head, a condition increasingly identified in older fighters, several physicians said.

The symptoms -- slurred speech, tremor, rigidity -- became progressively worse in Ali's case until it became necessary in 1984 for him to begin taking L-dopa, the main drug used to combat Parkinson's disease. "He responds very nicely to medication," Cope said. "That is the only drug he takes."

Although the damage slows his movement, "we have no evidence of other damage to the brain," Cope said, adding that Ali's mental abilities are normal.

"{The Mexican operation} is a revolutionary procedure," said Columbia's Fahn. "It has not been done on people who have not had Parkinson's disease. Whether it would work on Ali is not certain."

Dr. Abraham Lieberman, a neurosurgeon at New York University Medical Center and another pioneer in this experimental surgery -- he performed his first transplant last week and is expected to perform another today -- said he would be surprised if it helped Ali.

"I think there is too much injury to other areas," Lieberman said, but added that this is an experimental procedure that the Mexicans have tried on patients without classic Parkinson's disease and gotten some effect. "If you had asked me a year ago if an implant would work {in Ali}, I would say absolutely not. Today, I don't know. But I do not think it will work because he has more than just Parkinson's disease.

"I personally would not do an implant on Muhammad Ali at this time knowing what I know about the operation," Lieberman said.

It is not surprising that Ali would want to learn about the procedure, Fahn said. "This man has been articulate, a handsome man, a spokesman for his people and his causes. Parkinson's disease affects the motor system and it does slow people down." Said Fahn: "To have this happen has been a big blow to him."