For 2 to 3 days, patients will be admitted to the NIH Clinical Center for a levodopa (a dopamine agonist) dose-finding procedure. For this procedure, patients stop taking Sinemet and instead have levodopa, and subsequently apomorphine, infused through a vein. During the infusions, the drug dose is increased slowly until either 1) parkinsonian symptoms improve, 2) unacceptable side effects occur, or 3) the maximum study dose is reached. Symptoms are monitored frequently to find the optimal dose. (Patients who have had dosing infusions in the last 3 months will not have to undergo this phase of the study.)