Biotransformation of ftorafir (FT) was studied using 2-14C-ftorafur and 2;5'-14C-ftorafur. Both pyrimidine and tetrahydrofurane moieties of the FT underwent degradation to CO2 in rats. The cleavage of the pseudoglycosidic bonds C-N and formation of 5-fluorouracil was one of the steps of the FT metabolism that limited the velocity of the whole biotransformation process. Liver tissue NADPH-linked monooxygenating system was shown to participate in the process. The data obtained suggest that the role of enzymes involved in metabolism of nucleic acids was not significant in the FT metabolism under the conditions studied in vivo.