Patients who had therapeutic plasma levels of phenobarbital and/or diphenylhydantoin had significantly lower plasma levels of haloperidol and mesoridazine, the active metabolite of thioridazine, than patients who did not receive anticonvulsants. Plasma thioridazine levels per se were not affected by concomitant anticonvulsant treatment. Biperiden, an antimuscarinic, antiparkinsonian agent, did not affect the plasma levels of these three neuroleptics.