The metabolism of daunorubicin (DNR) to daunorubicinol and daunomycinone in murine leukaemia cells has been examined by means of high-performance liquid chromatography. A rapid and efficient extraction method has been developed that permits the recovery of the drug and its metabolites from cell homogenates. By means of high-performance liquid chromatography daunorubicinol and daunomycinone have been separated from DNR and the intracellular concentration of the compounds determined. The method developed is very rapid and sensitive, and amounts as small as 30 pg of DNR can be detected. The results indicate that the aldo-keto reductase is not very active in L1210 cells in culture, the main intracellular product found being DNR.