Patients enrolled in the study will, if possible, stop taking all antiparkinsonian medications except levodopa (Sinemet) for one month before the study begins and throughout its duration. For the first 1 to 3 days, patients will be admitted to the NIH Clinical center to undergo a levodopa "dose-finding" procedure. For this study, patients will stop taking Sinemet and instead will have levodopa infused through a vein for up to 8 hours/day. During the infusions, the drug dose will be increased slowly until either 1) parkinsonian symptoms improve, 2) unacceptable side effects occur, or 3) the maximum study dose is reached. Symptoms will be monitored frequently to find two infusion rates: 1) one that is less than what is needed to relieve symptoms, and 2) one that relieves symptoms but may produce dyskinesias. This procedure will be repeated at the end of weeks 2, 4 and 6 of the study.