A high-performance liquid chromatographic (HPLC) assay was developed for the determination of trazodone and its metabolite, 1-m-chlorophenylpiperazine (m-CPP), in plasma. The high level of trazodone in plasma was detected by ultraviolet absorbance at 254 nm and the low level of m-CPP in plasma was detected by coulometric electrochemical detection at 840 mV on the series arrangement of two detectors. Pilsicainide as an internal standard for both compounds was monitored by both detectors. Trazodone and m-CPP in plasma were extracted by a rapid and simple procedure based on CN bonded-phase extraction, and C8 reversed-phase HPLC separation. Determination was possible for trazodone in the concentration range 100-2000 ng mL-1 and for m-CPP in the concentration range 5-100 ng mL-1. The recoveries of trazodone and m-CPP added to plasma were 81.0-84.2 and 68.0-73.2%, respectively, with coefficients of variation of less than 7.3 and 8.2%, respectively. The method is applicable to high level monitoring of trazodone and low level monitoring of m-CPP in plasma of healthy volunteers and patients treated with trazodone.