Nine pyrimidine nucleoside analogues, in which the group attached at N-1 is a six-membered ring containing two heteroatoms, have been synthesized using the Vorbrüggen and Bennua (Vorbrüggen, H.; Bennua, B. Tetrahedron Lett. 1978, 1339) coupling procedure. These are 1-(1,4-oxathian-3-yl)-5-fluorouracil (8), 1-(4-oxo-1,4-oxathian-3-yl)-5-fluorouracil (two stereoisomers 9 and 10, resolved by silica gel column chromatography, 1-(1,4-oxathian-3-yl)-5-fluorouracil (11), 1-(1,4-oxathian-2-yl)-5-fluorouracil (12), 1-(1,4-dithin-2-yl)-5-flourouracil (15), 1-(1,4-dithian-2-yl)uracil (16), 1-(1,4-dithian-2-yl)thymine (17), and 1-(1,4-dioxan-2-yl)-5-fluorouracil (20). All of the analogous were tested for cell-growth inhibition using mouse and human tumor cell lines. The ID50 values of all of the analogues are greater than 10(-4) M, except in the case of 11 using the L1210 cell line. The most active analogues found are compounds 11 and 17, which were found to be 100 and 200 times less active, respectively, than 5-fluorouracil in the human erythroleukemia cell line, K-562.