1-(3-O-Benzyl-2-deoxy-beta-D-ribofuranosyl)-5-trifluoromethyl-2,4(1H,3)- pyrimidinedione (FTC-092), a fluorinated pyrimidine derivative, appeared to be effective against various transplantable tumors in mice following oral administration, and its activity was superior to that of several other antitumor fluorinated pyrimidines. The ED50 value for FTC-092 the dose effective in achieving 50% inhibition of tumor growth against the solid form of sarcoma 180 was 13.3 mg/kg daily, whereas those for 5-trifluoromethyl-2'-deoxyuridine (CF3dUrd), the parent compound of FTC-092, for 1-(2-tetrahydrofuryl)-5-fluorouracil (Tegafur, FT), the prodrug of 5-fluorouracil (FUra), and for FUra were 64.1, 122, and 28 mg/kg daily, respectively. The therapeutic indices (LD10/ED50) of FTC-092, CF3dUrd, FT, and FUra were 4.39, 1.7, 1.35, and 1.65, respectively. FTC-092 itself is not an active agent. After it has been absorbed from the gastrointestinal tract, FTC-092 undergoes a gradual biotransformation, mainly via the action of liver microsomes, releasing CF3dUrd over a long period. The levels of CF3dUrd in the stomach and small intestine of mice after the oral administration of FTC-092 were undetectable, whereas those following the administration of CF3dUrd at the same dose were high for a period of several hours. In contrast, the CF3dUrd level generated in plasma after the administration of FTC-092 remained at a high level for a longer period than did that observed on the administration of CF3dUrd. The low levels of CF3dUrd measured in stomach and small-intestine tissues and the maintenance of CF3dUrd in blood over long periods after the administration of FTC-092 are features that favor the possible clinical application of FTC-092.