A rapid and sensitive high-performance capillary electrophoretic method for the determination of clozapine and its main metabolite desmethylclozapine in human plasma was developed. The separation of the two analytes was carried out in an untreated fused-silica capillary [33 cm (8.5 cm effective length) x 50 microm I.D.] filled with a background electrolyte at pH 2.5 containing beta-cyclodextrin. Baseline separation of clozapine and desmethylclozapine was recorded in less than 3 min. An accurate sample pretreatment by means of solid-phase extraction and subsequent concentration allows for reliable quantitation of clozapine in the plasma of schizophrenic patients under treatment with the drug. The method showed good precision (mean RSD = 4.0%) as well as satisfactory extraction yields (approximately 88%) and a good sensitivity (limit of quantitation = 0.075 microg ml(-1), limit of detection = 0.025 microg ml(-1)).