Patient description: The patient is a 55-year-old man who was recently diagnosed with Parkinson's disease. He is complaining of slowness of movement and tremors. His disease is ranked as mild, Hoehn-Yahr Stage I. His past medical history is significant for hypertension and hypercholesterolemia. He lives with his wife. They have three children. He used to be active with gardening before his diagnosis. He complains of shaking and slow movement. He had difficulty entering through a door, as he was frozen and needed guidance to step in. His handwriting is getting smaller. He is offered Levodopa and Trihexyphenidyl. He is an alert and cooperative man who does not have any signs of dementia. He does not smoke or use any illicit drugs.

Clinical trial eligibility criteria: eligible ages (years): 0.0-999.0,  Parkinson Disease Healthy Patients: Patients with established, dopa-responsive PD, Hoehn and Yahr stage I-III, age 21 and older will be included. Patients will be recruited from the NINDS HMCS and ETB clinical programs and from the community. Healthy Controls: Healthy controls 21 and older will be recruited through the NIH Clinical Research Volunteer Program and from the community  Exclusionary will be: any significant medical or psychiatric illness (other than PD), pregnancy, history of epilepsy, concurrent use of tricyclic antidepressants, neuroleptic agents, or any other licit or illicit drugs that could lower the seizure threshold or significantly affect cortical excitability. Persons with surgically or traumatically implanted foreign bodies in the head (other than dental appliances of fillings) that may pose a physical hazard during magnetic stimulation will also be excluded. Patients: Patients with persistent "resting" tremor that precludes testing at rest may be excluded as well as any patient to whom the screening physician makes a judgment that withdrawal of medications poses a significant risk due to immobility or risk of falling. Patients on long-acting dopamine agonists will also be excluded. Patients in Hoehn and Yahr stage IV will also be excluded, since they are least likely to tolerate medication withdrawal and are also unlikely to be able to maintain muscle relaxation for TMS testing. No significant medical or psychiatric illness (other than PD), pregnancy, history of epilepsy, concurrent use of tricyclic antidepressants, neuroleptic agents, or any other licit or illicit drugs that could lower the seizure threshold or significantly affect cortical excitability. Must not have surgically or traumatically implanted foreign bodies in the head (other than dental appliances or fillings) that may pose a physical hazard during magnetic stimulation. Ages 21 and older will be recruited through the NIH Clinical Research Volunteer Program (CRVP) and from the community. All individuals will be processed through the CRVP before participating. Recruitment will be focused on optimizing age, sex, educational, and racial matching with patient group. Patients' spouses will be invited to participate. No significant medical or psychiatric illness (other than PD), pregnancy, history of epilepsy, concurrent use of tricyclic antidepressants, neuroleptic agents, or any other licit or illicit drugs that could lower the seizure threshold or significantly affect cortical excitability. Must not have surgically or traumatically implanted foreign bodies in the head (other than dental appliances or fillings) that may pose a physical hazard during magnetic stimulation