['To investigate the effect of valproic acid on plasma levels of risperidone and its active metabolite, 9-hydroxyrisperidone under steady state conditions in 12 schizophrenic patients.', 'The efficacy and tolerability for the combination treatment of valproic acid and risperidone were examined.', 'The addition of valproic acid to risperidone significantly reduced total scores of PANSS positive symptoms, especially excitement and hostility scores, but did not change SAS scores. Addition of valproic acid did not alter plasma concentrations of risperidone or 9-hydroxyrisperidone or active moiety, and the risperidone/9-hydroxyrisperidone ratio. The combination of valproic acid with risperidone decreased plasma levels of HVA, but not those of MHPG; additionally, treatment with this combination was found to reduce dopaminergic activity.'] Does valproic acid improve psychotic agitation without influencing plasma risperidone levels in schizophrenic patients?