and O-ethyl derivatives of 2'-deoxy-5-(trifluoromethyl)uridine (F3Thd) and 2'-deoxy-5-fluorouridine (FUdR) were synthesized. The oral antitumor activity of the compounds against sarcoma 180 in mice was examined. The 5'-O-ethyl (3b), 3'-O-ethyl (3c), 5'-O-benzyl (3e), and 3'-O-benzyl (3f) derivatives of F3Thd were 4-fold more active than F3Thd itself. Among the substituted-benzyl derivatives of F3Thd,