Patients between 18 and 75 years old who have moderately advanced Parkinson's disease and motor problems resulting from levodopa therapy may be eligible for this 5-week study. Candidates will have a complete medical history and physical examination with a detailed neurological evaluation. If needed, some patients will undergo a magnetic resonance imaging (MRI) or computerized tomography (CT) scan of the brain and a chest X-ray. All patients will have blood and urine tests and an electrocardiogram (EKG) and will take a written test for evaluation of depression.