Participants will have a medical history, physical examination, detailed neurologic evaluation, routine blood tests, urinalysis and an electrocardiogram. A chest X-ray and MRI or CT scan of the brain will be done if needed. In addition, an ACTH stimulation test will be done before and at the end of the study. For this test, a hormone called ACTH is injected into a vein. A small amount of blood is drawn before the injection and 30 and 60 minutes afterwards to measure levels of another hormone called cortisol. After these tests are completed, patients will, if possible, stop taking all antiparkinsonian medications except levodopa (Sinemet) for one month before the study begins and throughout its duration.