A new and sensitive HPLC method has been developed for the determination of captopril plus its disulfide metabolites (total captopril) in human plasma. Captopril disulfides and the drug covalently bound to protein were reduced with sodium borohydride to captopril. After liquid-liquid extraction, captopril was treated with o-phthalaldehyde in the presence of D-phenylalanine. The fluorescent derivative of captopril was measured by HPLC using a C-18 reversed phase column with fluorescence detection at the excitation and emission wavelengths of 235 nm and 440 nm, respectively. The mobile phase consisted of a methanol-acetonitrile-phosphate buffer (0.02 mol.L-1, pH 6.4) mixture (30:30:135, v/v), and was set at a flow rate of 1 ml.min-1. The linear range of the assay was between 5 ng.ml-1 (lower limit of quantitation) and 300 ng.ml-1 for total captopril in plasma. The method was successfully applied to determine plasma concentrations of captopril plus its disulfide metabolites in hypertensive patients and was demonstrated to be suitable for the therapeutic drug monitoring.