Traditionally, levodopa has been given as a pill. In this study, researchers would like to inject levodopa directly into a vein (intravenous) in order to diagnose cases of Parkinson's disease. This method provides immediate results and allows doctors to adjust the dose of levodopa very carefully. Because the intravenous method of giving levodopa is less practical than oral medication, it is not an available alternative for the routine treatment of Parkinson's disease. However, it may be useful for faster diagnosis of the disease and for determining effective doses of oral medication.