To test the effect of changes in electronegativity within the alicyclic N-1 substituent 5-fluorouracil analogues on cytotoxic activity, a series of derivatives of ftorafur, 1-(2'-tetrahydrofuranyl)-5-fluorouracil, was synthesized and tested for antitumor activity in the P388 lymphocytic leukemia screen and cytotoxic activity in the L1210 cell culture screen. Two compounds of N-1 substituent with high electronegativity, the 2'-tetrahydrothiophene 1'-oxide and the 2'-tetrahydrothiophene 1',1'-dioxide derivatives, demonstrated the highest in vitro L1210 cell inhibition (84.5% and 92.0%, respectively). Furthermore, against P388 lymphocytic leukemia in vivo, the 2'-tetrahydrothiophene 1'-oxide derivative showed significant activity (T/C = 143). Other compounds of similar or lower electronegativity within the N-1 cyclic substituent were inactive against P388 lymphocytic leukemia and less active against L1210 cells.