A gas-chromatographic method was used for the study in man of the kinetics of perphenazine (PPZ) and its sulphoxide metabolite (PPZ-SO). Various forms of PPZ administration were applied in eighteen schizophrenic patients and four healthy volunteers. Following an i.v. dose of 5 or 6 mg a considerable fluctuation in the plasma concentration was noted before the exponential elimination phase. The average terminal half-life of PPZ was approximately 9.5 hours. PPZ-SO showed up quickly but in low concentrations. After an oral dose of 6 mg no PPZ was detected in plasma and PPZ-SO only as traces. During continuous oral medication, 12 mg three times daily, a low systemic availability and a high PPZ-SO/PPZ ratio was found suggesting a marked first pass effect. PPZ-enanthate given i.m. fortnightly resulted in PPZ-levels comparable to those seen after continuous oral medication, but PPZ-SO concentration were much lower. No accumulation was observed. The systemic clearance rate (average approximately 100 1/h) was the same after PPZ-enanthate i.m. and PPZ i.v., but varied three-fold individually. Side effects were mostly, but not always, registered concomitant with high plasma levels of PPZ.