For 1 to 3 days, patients will be admitted to the NIH Clinical center to undergo a levodopa and apomorphine (a dopamine agonist) "dose-finding" procedure. For this procedure, patients will stop taking Sinemet and instead will have levodopa, and subsequently apomorphine, infused through a vein. 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.