1. The case of a schizophrenic patient taking bromperidol (18 mg/day), whose psychotic symptoms deteriorated markedly after addition of cisapride (7.5 mg/day), is presented. 2. Retrospective determination of drug concentrations revealed that plasma concentrations of bromperidol and its reduced metabolite were increased after cisapride addition. 3. The present study thus suggests that there is an interaction between cisapride and bromperidol.