• The relationship between clinical response of schizophrenic patients to haloperidol and (1) blood levels of the medication, determined by both gas-liquid chromatography (GLC) and radioreceptor (RR) assays, or (2) prolactin response to the medication, was examined in an inpatient study using several fixed doses of haloperidol. Regression analysis disclosed a substantial curvilinear relationship between steadystate GLC-determined plasma haloperidol levels and decrease in Brief Psychiatric Rating Scale (BPRS) Psychosis factor scores; however, no substantial relationship was found between clinical response and RR plasma haloperidol levels or serum prolactin response to haloperidol. Our results suggest that steady-state plasma levels of haloperidol determined by the GLC chemical assay are a better predictor of decreases in BPRS Psychosis factor scores than RR assayed plasma haloperidol levels or prolactin response to haloperidol.