We studied the effects of transcranial magnetic stimulation (TMS) of the motor cortex on simple reaction time (RT) in 10 patients with Parkinson's disease compared with 10 age-matched normal controls. The subjects flexed their right elbow rapidly in response to a visual go-signal. In random trials, TMS was applied to the left motor cortex at varying delays after the go-signal. In trials without TMS, RT was longer in the patients. However, in the trials with subthreshold TMS, RT in the patients became as fast as RT in trials without TMS in the controls. This shortening was associated with normalization of the voluntary triphasic EMG pattern and the pre-movement cortical excitability increase.