Furan is a hepatic toxicant and carcinogen in rodents. Its microsomal metabolite, cis-2-butene-1,4-dial, is mutagenic in the Ames assay. Consistent with this observation, cis-2-butene-1,4-dial reacts with 2′-deoxycytidine, 2′-deoxyguanosine, and 2′-deoxyadenosine to form diastereomeric adducts. HPLC analysis indicated that the rate of reaction with deoxyribonucleosides was dependent on pH. At pH 6.5, the relative reactivity was 2′-deoxycytidine > 2′-deoxyguanosine > 2′-deoxyadenosine whereas it was 2′-deoxyguanosine > 2′-deoxycytidine > 2′-deoxyadenosine at pH 8.0. Thymidine did not react with cis-2-butene-1,4-dial. The primary 2′-deoxyguanosine and 2′-deoxyadenosine reaction products were unstable and decomposed to secondary products. NMR and mass spectral analysis indicated that the initial 2′-deoxyadenosine and 2′-deoxyguanosine reaction products were hemiacetal forms of 3-(2′-deoxy-β-Derthyropentafuranosyl)- 3,5,6,7-tetrahydro-6-hydroxy-7- (ethane-2″-al)-9H-imidazo[1,2-(xlpurine-9-one (structure 2) and 3-(2′-deoxy-β-D-erythropentafuranosyl)- 3,6,7,8-tetrahydro-7-(ethane-2″-al)-8-hydroxy-3H-imidazo[2,1-i]purine (structure 4), respectively. These adducts resulted from the addition of cis-2-butene-1,4-dial to the exo- and endocyclic nitrogens of 2′-deoxyadenosine and 2′-deoxyguanosine. The data provide support for the hypothesis that cis-2-butene-1,4-dial is an important genotoxic intermediate in furan-induced carcinogenesis.