A method was developed for the determination of naftopidil (NAF) and its main metabolite desmethyl-naftopidil (DMN) in plasma by HPLC. A mixture of methanol, acetonitrile and 0.02 mol.L-1 K2HPO4 solution was used as mobile phase. Stationary phase was RP-C18. Propranolol was chosen as internal standard (IS), and the detector wavelength was 240 nm. Under these conditions, the retention times of IS, DMN and NAF were 6.2, 7.6 and 10.5 min respectively. Absolute recoveries from plasma were up to 93.4% (IS), 76.8% (DMN) and 80.2% (NAF). The assay was linear for DMN and NAF in the range of 10-800 ng.ml-1 plasma concentration, gamma = 0.9998. The lowest detection limits were 8 ng.ml-1 (DMN) and 5 ng.ml-1 (NAF), and the precisions within-day and day-to-day were no more than 6.3% for MDN and 11.6% for NAF. The drug concentration-time pattern of these two substances was also studied with this method. Result showed that naftopidil was quickly absorbed after p.o. and the concentration-time data were fitted to two compartment model, with half life (T1/2 beta) of 8.13 h.