This study will evaluate the effects of an experimental drug called EMD 128130 on Parkinson's disease symptoms and on dyskinesias (involuntary movements) that develop as a result of long-term treatment with levodopa. EMD 128130 inhibits the function of serotonin, a chemical messenger thought to regulate dopamine release, and thus affect Parkinson's disease symptoms.

Patients with relatively advanced Parkinson's disease between 30 and 80 years of age may be eligible for this 3-phase study.

• Phase 1 - Baseline Evaluation

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.

• Phase 2 - Levodopa and Apomorphine Dose Finding

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.

• Phase 3 - Active Study

Patients will begin the treatment phase within 3 months of the dose-finding phase. After a brief physical examination, routine blood tests, and evaluation of parkinsonism symptoms, patients will begin taking either EMD 128130 tablets or capsules or a placebo (a look-alike pill with no active ingredient) twice a day, along with Sinemet, for 3 weeks. All participants will receive placebo at least 1 week during the study, and about four patients, selected at random, will receive only placebo the entire 3 weeks. Levodopa and apomorphine infusions will be repeated at the end of weeks 1, 2 and 3 of Phase 3. The procedure for the infusions will be the same as in the dose-finding phase.

Throughout the study, parkinsonian symptoms and dyskinesias will be assessed and blood samples will be drawn periodically to measure drug levels. Patients will return for a follow-up evaluation 2 weeks after the end of the study.