4'-C-Ethynyl-beta-D-arabino- and 4'-C-ethynyl-2'-deoxy-beta-D-ribo-pentofuranosylpyrimidine and -purine nucleosides were synthesized and evaluated for their in vitro anti-HIV activity. The key intermediate, 4-C-ethynyl- or 4-C-triethylsilylethynyl-D-ribo-pentofuranose, was prepared from D-glucose and glycosidated with various pyrimidine or purine bases. The arabinopyrimidine derivatives were prepared from the corresponding ribo derivatives via O(2),2'-anhydro nucleosides. The 2'-deoxy-ribo derivatives were synthesized by radical reduction of 2'-bromo or 2'- phenoxythiocarbonyloxy nucleosides. Among these 4'-C-ethynyl nucleosides, seven analogues proved to be potent against HIV-1 in vitro with EC(50) values ranging from 0.0003 to 0. 03 microM. These compounds also exerted activity against clinical and multi-dideoxy-nucleoside-resistant HIV-1 strains with comparable EC(50) values. Three such 4'-C-ethynyl-2'-deoxypurine analogues including 4'-C-ethynyl-2'-deoxyadenosine and 4'-C-ethynyl-2, 6-diamino-2'-deoxypurine were less cytotoxic [selectivity indices (SIs): 975-2733] than three 4'-C-ethynyl-2'-deoxycytidine analogues (SIs: 63-363). 4'-C-Ethynyl-5-fluoro-2'-deoxycytidine was least toxic (SI: >3333) and potent against all HIV strains tested.