An experimental brain operation to treat Parkinson's disease has produced moderate improvement for some victims but not the "miracle" that an earlier report suggested, a researcher said.

"All the patients in our group have said they felt it was worthwhile, but in no case has it achieved what they hoped it would," said Dr. Christopher G. Goetz of Rush-Presbyterian-St. Luke's Medical Center in Chicago.The operation involves removing tissue from patients' adrenal glands and transplanting it in their brains. The goal is to provide the victims' brains with a source of dopamine, an essential message-carrying chemical that they lack.

The operation was first reported two years ago by Dr. Ignacio Madrazo and colleagues at the National Autonomous University of Mexico and the social security hospital system in Mexico City. They reported dramatic improvement in two patients. Since then, surgeons at many medical centers have tried to duplicate their results.