The response times in choice-reaction tasks are faster when the relative spatial positions of stimulus and response match than when they do not match, even when the spatial relation is irrelevant to response choice. This spatial stimulus-response (S--R) compatibility effect (i.e., the Simon effect) is attributed in part to the automatic activation of spatially corresponding responses, which need to be suppressed when the spatial location of stimulus and correct response do not correspond. The present study tested patients with Parkinson's disease and healthy control subjects in a spatial S--R compatibility task in order to investigate whether basal ganglia dysfunction in Parkinson's disease leads to disinhibition of direct visuomotor activation. High-density event-related brain potential recordings were used to chart the cortical activity accompanying attentional orientation and response selection. Response time measures demonstrated a failure to inhibit automatic response activation in Parkinson patients, which was revealed by taking into account a sequence-dependent modulation of the Simon effect. Event-related potential (ERP) recordings demonstrated that visuospatial orientation to target stimuli was accompanied by signal-locked activity above motor areas of the cortex, with similar latencies but an enhanced amplitude in patients compared to control subjects. The results suggest that inhibitory modulation of automatic, stimulus-driven, visuomotor activation occurs after the initial sensory activation of motor cortical areas. The failed inhibition in Parkinson's disease appears therefore related to a disturbance in processes that prevent early attention-related visuomotor activation, within motor areas, from actually evoking a response.