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Plasma and red blood cell levels of haloperidol, thioridazine, and thioridazine's main metabolite mesoridazine were measured in schizophrenic outpatients during treatment with fixed doses of haloperidol or thioridazine for several months. These drug levels were compared to those in schizophrenic inpatients treated with fixed doses of the same neuroleptics. There were large interpatient variations in plasma and red blood cell levels at a given dose for schizophrenic outpatients as well as for inpatients. The intrapatient day-to-day fluctuation was much greater in the outpatients. The mean coefficient of variation of thioridazine or mesoridazine levels was about two-fold higher in schizophrenic outpatients than in inpatients. Differences in blood sampling time or compliance in medication ingestion do not fully explain the issue. The factors accounting for the increased intrapatient variability of plasma levels of thioridazine, mesoridazine, and haloperidol in schizophrenic outpatients remain unclear.