Studies with CGP 41 251 (I), an N-benzoylstaurosporine derivative and PKC-alpha inhibitor, revealed that oral administration of 400 microg/day of the compound to wild type mice on four successive days reversed multi drug resistance (Killion et al. Oncology Research 7: 453-459, 1995). In our study, the same regimen of administration was followed with the primary objective to establish the pharmacokinetics and metabolism of the compound and to substantiate at which plasma concentrations of CGP 41 251 multi drug resistance (MDR) reversal can be expected. Concentrations of CGP 41 251 and metabolites in plasma were determined by a validated high performance liquid chromatography (HPLC) method with fluorescence detection. Structural characterization of the metabolites was performed with HPLC and mass spectrometric detection. In our experiment extensive metabolism of CGP 41 251 was found. The presence of five hydroxylated metabolites of CGP 41 251 (I) was confirmed and two metabolites were structurally elucidated as CGP 50 750 (III) and CGP 52 421 (V). Maximal concentrations of 73 ng/ml, 1.9 ng/ml and 126 ng/ml for CGP 41 251 (I), III and V were found, respectively. The mass spectra of the other three metabolites indicate that these are oxidized nitrogens or hydroxylated compounds. As yet, the oxidation or hydroxylation sites have not been established. This study has revealed new information about CGP 41 251 pharmacokinetics and metabolism. Target levels between 10-100 ng/ml may be important to achieve in further clinical trials with CGP 41 251 as MDR modulator.