Although much of the excitement about the fight against Parkinson's disease focuses on the brain tissue transplant experiments, a large national trial is now getting under way to determine whether two drugs can slow this progressive disorder.

Twenty-eight medical centers in the United States and Canada have joined a five-year, $10 million study of the ability of Deprenyl and tocopherol -- a chemical cousin of vitamin E -- to protect dopamine-producing neurons in the brain from the progressive loss associated with this disease. They will be tested individually and together.

Parkinson's "is a disease of aging," said Dr. Ira Shoulson of the University of Rochester Medical Center and chief of the study, sponsored by the National Institute of Neurological and Communicative Disorders and Stroke. But it increasingly seems to be striking younger patients.

Although the disease can be treated with L-dopa -- a dopamine building block -- the drug fails to help 15 percent of the Parkinson's patients and eventually causes such severe side effects -- including uncontrollable freezing episodes and hallucinations -- that it cannot be used. "Eventually," Shoulson said, "all patients show a loss of {L-dopa's} effect. At 10 years, you see a loss of symptomatic effectiveness in virtually all patients."

Deprenyl has been used in Europe for some time, but there have not been satisfactory studies to determine whether it will protect the dopamine-making neurons.

Recent work suggests that dopamine neurons are killed by toxic chemicals in the environment. The studies suggest that Deprenyl helps protect against these toxins.