5-Ethyl-2'-deoxyuridine (EtUdR) was converted with rat liver extract, rat liver cells and rat liver cell cultures into its metabolites 5-ethyluracil and 5-(1-hydroxyethyl) uracil. The analytical identification of the metabolites as well as the investigation of the biotransformation process was performed by thin layer chromatography, autoradiography and high pressure liquid chromatography.