Patients will stop taking all anti-parkinsonism medications except levodopa (Sinemet) and the experimental drug during the study. For the first 1 to 3 days, patients will be in the hospital for a levodopa "dose-finding" procedure. For this study, levodopa is infused through a vein for up to 8 hours, with symptoms monitored frequently to determine the doses that will produce two results: 1) the dose that is less than what is needed to relieve symptoms, and 2) the dose that relieves symptoms, but may produce dyskinesias.