A high-performance liquid chromatographic method with electrochemical detection has been developed for the simultaneous determination of epirubicin, 13-S-dihydroepirubicin, doxorubicin and 13-S-dihydrodoxorubicin in human plasma. An aliquot of 200 microl plasma, spiked with internal standard, was extracted by solid-phase extraction using polymeric adsorbent columns. Chromatography was performed using a C18 reversed-phase column with a mobile phase consisting of water-acetonitrile (71:29, v/v) containing 0.05 M Na2HPO4 and 0.05% v/v triethylamine adjusted to pH 4.6 with citric acid. Linearity of the method was obtained in the concentration range of 1-500 ng/ml for all the analytes. Analytical recoveries of the analytes ranged from 89 to 93%. The assay can be used for the simultaneous determination of the four analytes, or for epirubicin and its metabolite or doxorubicin and its metabolite, using the other parent drug as an internal standard. The method was applied to analyze human plasma samples from patients treated with epirubicin using doxorubicin as an internal standard.