A series of 4'alpha-C-branched-chain pyrimidine nucleosides was synthesized from 2'-deoxycytidine or uridine. In the 2'-deoxycytidine series, the substituent at the 4'alpha-position affected cytotoxicity against L1210 mouse leukemic cells in vitro in the order Me (23) > CN (22) > C(symbol)CH (21) > CH=CH(2) (19) > Et (24) > CH=CHCl (20). However, uridine and cytidine derivatives with ethynyl and cyano groups at the 4'alpha-position did not show any cytotoxicity. The antiviral activities of these nucleosides against HSV-1, HSV-2, and HIV-1 in vitro were also examined. Compounds 22 and 23 showed antiviral activities against HSV-1 and HSV-2 without showing significant toxicity to the host cells (MRC-5 cells). Although almost all of the nucleosides showed anti-HIV-1 activities, they were also cytotoxic to the host cells (MT-4).