Risperidone is a novel and clinically effective atypical antipsychotic medication with a unique biochemical profile. To contrast the neurophysiological effects of this new medication with those of a typical antipsychotic medication, we performed quantitative measurements of saccadic eye movements in a series of antipsychotic-naive schizophrenic patients treated with either risperidone or haloperidol. Patients were tested before and after 1 month of treatment, and a matched group of healthy subjects was tested twice over a similar time interval. Risperidone, but not haloperidol, was associated with prolonged latency and decreased peak velocity and accuracy of saccadic eye movements that was detectable 4 weeks after treatment initiation. The adverse effects of risperidone may be due to the lack of development of acute tolerance to its powerful serotonergic (5-HT2A) antagonism, which could be responsible for the disruption of brainstem physiology in regions controlling saccadic eye movements.