This report describes the synthesis of unsaturated exomethylene lyxopyranonucleoside analogues as potential biologically active agents. Commercially available 1,2,3,4-tetra-O-acetyl-alpha-D-lyxopyranose 1 was condensed with silylated thymine and uracil, respectively, deacetylated and acetalated to afford 1-(2,3-O-isopropylidene-alpha-D-lyxopyranosyl)thymine 4a and 1-(2,3-O-isopropylidene-alpha-D-lyxopyranosyl)uracil 4b. The new derivatives 1-(2,3,4-trideoxy-4-methylene-alpha-pent-2-enopyranosyl)thymine 8a and 1-(2,3,4-trideoxy-4-methylene-alpha-pent-2-enopyranosyl)uracil 8b were prepared via two different key intermediates, 7a, b and 13a, b in order to elucidate the influence of 2',3'-unsaturation and to clarify the difference between the keto and exomethylene group on the biological activity of the target molecules. Compounds 7a, b, 8a, b, and 13a, b were evaluated for their antiviral and cytostatic activity using several virus strains and cell lines. Whereas no marked antiviral activity was noticed, 13a and 13b showed a cytostatic activity that ranged between 7 and 23 muM for 13a and 26 and 38 muM for 13b against murine leukemia L1210, human lymphocyte Molt4/C8 and CEM cells, and human breast carcinoma MCF7 cells.