New carbocyclic analogs (12, 14, and 15) of 5-fluorouracil nucleosides have been prepared by fluorination, using elemental fluorine, of the acetylated analogs (8, 10, and 11, respectively) of 1-(2′,3′-dideoxy-glycero-pentofuranosyl)-and 1-(2′- and 3′-deoxy-β-threo-pentofuranosyl)-uracil nucleosides. The carbocyclic analog (14) of 1-(2′-deoxy-β-threo-pentofuranosyl)-5-fluorouracil was then utilized for the preparation of various new carbocyclic analogs of 1-(3′-substituted-2′,3′-dideoxy-β-erythro-pentofuranosyl)-5-fluorouracil nucleosides by SN2 displacement of the 3′-mesyloxy group in the protected derivative 17. In this way analogs having the chloro (see 23), bromo (see 24), and azido (see 25) groups at the 3′-position were obtained; the analog 29 having a 3′-amino group was obtained by reduction of the azido group. In addition (±)-5-fluoro-1-{3-(hydroxymethyl)-3-cyclopenten-1-yl}-2,4(1H,3H)-pyrimidinedione (26) was obtained as an elimination side-reaction product in the preparation of the 3′-chloro and 3′-bromo analogs. Compounds 8, 9, 12, 14, 15, 25, and 29 were tested against the herpes simplex viruses and were found to be inactive. Compound 26 inhibited the growth of K-562 cells with an ID50 (10−4–10−3), which was about 100-fold greater than that of 5-fluorouracil in this test system. Analogs 12, 25, and 29 were slightly cytotoxic.