A number of nucleoside and nucleotide derivatives of 4-hydroxy-3-beta-D-ribofuranosylpyrazole-5-carboxamide (pyrazofurin, 1) were prepared and tested for their antiviral and cytostatic activity in cell culture. Treatment of 1 with benzyl bromide gave 4-O-benzylpyrazofurin (4). Methylation of 4 with CH2N2 and subsequent removal of the benzyl group by catalytic hydrogenation provided 1-methylpyrazofurin (8). Direct methylation of 1 with CH3I furnished 4-O-methylpyrazofurin (6). Dehydration of the pentaacetylpyrazofurin (9) with phosgene furnished 4-acetoxy-3-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl)-1-acetylpyrazol e-5-carbonitrile (10). A similar dehydration of the precursor tetraacetyl derivative of 4 gave the corresponding carbonitrile, which on deprotection and subsequent treatment with hydroxylamine furnished 4- (benzyloxy)-3-beta-D-ribofuranosylpyrazole-5-carboxamidoxime (13). Treatment of the tetraacetyl derivative of 4 with Lawesson's reagent and subsequent deacetylation furnished a mixture of 4- (benzyloxy)-3-beta-D-ribofuranosylpyrazole-5-thiocarboxamide (15) and the corresponding nitrile derivative (16). Phosphorylation of unprotected 4 with POCl3 and subsequent debenzylation of the intermediate 17 gave pyrazofurin 5'-phosphate (18), which provided the first chemical synthesis of 18. Similar phosphorylation of 4 with POCl3 and quenching the reaction mixture with either EtOH or MeOH, followed by debenzylation, furnished the 5'-O-(ethyl phosphate) (19b) and 5'-O-(dimethyl phosphate) (20b) derivatives of pyrazofurin. DCC-mediated cyclization of 17, followed by debenzylation, gave pyrazofurin 3',5'-(cyclic)phosphate (21b). The NAD analogue 23b was also prepared by the treatment of 17 with an activated form of AMP in the presence of AgNO3. The structural assignment of 7,8, and 20a were made by single-crystal X-ray analysis, and along with pyrazofurin, their intramolecular hydrogen bond characteristics have been studied. All of these compounds were tested in Vero cell cultures against a spectrum of viruses. Compounds 18 and 23b were active at concentrations very similar to pyrazofurin but are less toxic to the cells than pyrazofurin. Compounds 19b, 20b, and the 3',5'-(cyclic)phosphate 21b are less active than 1. Compounds 18, 19b, 20b, and 23b also exhibited significant inhibitory effects on the growth of L1210 and P388 leukemias and Lewis lung carcinoma cells in vitro, whereas B16 melanoma cells were less sensitive to growth inhibition by these compounds. Pyrazofurin derivatives modified at the 1-, 4-, or 5-position showed neither antiviral nor cytostatic activity in cell culture.