Active study phase - Patients are randomly assigned to take levetiracetam or placebo ("sugar pill") twice a day for 6 weeks. At the end of weeks 1, 2 4, and 5, patients come to the clinic for blood tests, an EKG, and a review of adverse side effects. At the end of weeks 3 and 6, patients are hospitalized to study the response to treatment. They again stop taking Sinemet and selegiline and their ability to perform motor tasks is evaluated. They are then placed on an L-dopa infusion for 10 hours. Placebo may be infused at various times instead of L-dopa. Motor symptoms are evaluated several times during the infusion. Blood is drawn once during the infusion for research studies.