The event-related potential and motor reaction time were simultaneously recorded in 35 patients with Parkinson's disease (26 nondemented and nine demented) and 15 age-matched neurologically normal control subjects during the performance of visual discrimination tasks. There were no significant differences in either the latency or amplitude of the P300 component between the nondemented patients and the control subjects, but the patients with nondemented Parkinson's disease had a significantly prolonged reaction time compared with the controls. In patients with demented Parkinson's disease, both P300 latency and reaction time were significantly prolonged compared with the normal controls. These results suggest that response selection and execution are impaired in patients with nondemented Parkinson's disease, although the stimulus evaluation process is largely preserved, whereas patients with demented Parkinson's disease have impairment of stimulus evaluation, go/no-go response selection, and execution.