1 Response to chlorpromazine therapy was studied in 24 schizophrenics and 75% reduction in Brief Psychiatric Rating scale (BPRS) within 4 weeks of treatment was taken to indicate good response. 2 Test oral dose of CPZ (200 mg) was administered and serial blood samples and total urine excreted in 6 h were taken for analyses of CPZ and its metabolites. 3 Area under the plasma time curve (AUC) showed an inverted U-shaped relationship with % reduction in BPRS. 4 Three groups of patients were identified, responders, l- and h-group non-responders. 5 l- and h-groups respond on further treatment with haloperidol (30 mg daily) and ECT respectively. 6 Responders showed threshold metabolic molar fraction MMFHO-CPZ greater than or equal to 3.0 x 10(-5) and MMFHO-CPZ/MMFCPZ-SO greater than 1.