Levodopa-induced dyskinesia (LID) is a major source of disability of many patients with Parkinson's disease (PD). LID may be associated with increased activity of the part of the brain that mediates voluntary movement at the motor cortex and its interconnected areas. The human brain can be stimulated non-invasively with a technique know as transcranial magnetic stimulation (TMS). Low frequency repetitive TMS (rTMS) at about one per second decreases cortical excitability and previous studies suggested that rTMS may be effective in treating neurological and psychiatric disorders associated with increased cortical excitability such as depression, dystonia, schizophrenia and epilepsy. We hypothesize that low frequency motor cortex stimulation will produce lasting reduction in the excitability of the motor cortex and its interconnected areas and this will lead to improvement in LID in PD patients. We will be conducting an open labeled trial of low frequency rTMS for treatment of LID. Six PD patients with LID will receive a 10-day course of low frequency rTMS of the motor cortex. Dyskinesia will be assessed by the response to a dose of levodopa (levodopa challenge test) before, one day and 14 days after rTMS treatment. Cortical excitability will be measured with TMS immediately after each levodopa challenge test. Demonstration of a significant anti-dyskinetic effect of low frequency rTMS would support the need for further studies and this may become a useful adjunctive therapy for PD patients. The study will also advance our understanding of the pathophysiology of PD and LID which will be valuable in designing novel therapies for PD patients.