Associations between neuroleptic side effects and plasma concentrations of the drug and prolactin were investigated in 33 acutely exacerbated schizophrenic patients (16 males and 17 females) treated with a fixed dose of nemonapride (18 mg/day), a new substituted benzamide, for 3 weeks. The most frequently observed side effects during nemonapride treatment were extrapyramidal symptoms such as akathisia (69.7%), dystonia (48.5%), hypokinesia (45.5%), tremor (39.4%) and increased salivation (36.4%). There were positive correlations between prolactin response and extrapyramidal side effects (EPS) scores after 1 week (Spearman rank correlation rs=.651, P<.01), 2 weeks (rs=.567, P<.05) and 3 weeks (rs=.670, P<.01) in male patients although no significant correlations were found in female or total patients. No significant correlations were found between plasma concentrations of the drug and total or any subscale side effects scores. The present study thus suggests that the spectrum of nemonapride-induced side effects is characterized by predominant extrapyramidal symptoms, and that prolactin response as an index of dopamine blockade reflects severity of EPS at least in male patients treated with nemonapride.