The metabolic interaction between carbamazepine (CBZ) and haloperidol (HP) was studied in Japanese schizophrenic patients treated with HP but not with CBZ and with both CBZ and HP. The serum CBZ concentrations in patients treated without HP were significantly decreased (p< 0.05), on average 40%, as compared to those in patients treated with both CBZ and HP, whereas the serum HP concentrations in patients treated with both HP and CBZ were significantly decreased (p< 0.05), as compared to those in patients treated with HP but not with CBZ. The effect of HP, which prevents the serum CBZ level from decreasing, was shown in this study.