In this clinical trial, the safety of MK0657 when given with levodopa will be assessed in patients with Parkinson's Disease. This study will also measure the effectiveness of MK0657, when given in combination with levodopa, to improve motor symptoms and ameliorate dyskinesias (uncontrolled movements of a part of the body) as compared to placebo.

Inclusion Criteria:

• Parkinson's Disease patients between the ages of 40 and 80 inclusive
• Patient is in general good health based on screening assessments
• Patient is willing to discontinue anti-parkinson's medications at least 8 hours prior to dosing
• Patient is levodopa responsive with levodopa-induced peak-dose dyskinesias
• Patient taking selective serotonin inhibitors (SSRIs), sleep medications and neuroleptics, have been on a stable dose for at least 30 days and has not experienced any CNS-related side effects
• Patient is not a heavy smoker or drinker

Exclusion Criteria:

• Patient has atypical Parkinson's syndrome due to drugs, identified metabolic and/or neurologic disorders, encephalitis, or other degenerative syndrome
• Patient has known intolerance or hypersensitivity to levodopa or carbidopa
• Patient has been on anticholinergics or memantine within 30 days prior to dosing
• Patients have a history of the following: seizure disorder, stroke or head trauma, pronounced cognitive impairment, psychiatric disorder, cardiovascular disease, cancer, diabetes as defined by HbA1c of greater than 8%
• Patient has a systolic BP or less than 80 mm Hg or greater than 150 mm Hg