trans-4-Hydroxy-2-hexenal (t-4HH), a reactive metabolite isolated from the pyrrolizidine alkaloid senecionine, and trans-4-hydroxy-2-nonenal (t-4HN), a product of lipid peroxidation, reacted nonenzymatically with deoxyguanosine at pH 7.4 at 37 degrees C in vitro with each compound yielding two pairs of diastereomeric adducts. Adducts were isolated using reverse phase high-performance liquid chromatography and were characterized by their mass spectra and proton magnetic resonance spectra. Adducts 1 and 2 from t-4HH were assigned the structures 3-(2-deoxy-beta-D-erythro-pentofuranosyl)-5,6,7,8-tetrahydro-8R-hydroxy- 6S[1- (R and S)hydroxypropyl]pyramido[1,2-a]purine-10-(3H)one and Adducts 3 and 4 were assigned the structures 3-(2-deoxy-beta-D-erythro-pentofuranosyl)-5,6,7,8-tetrahydro-8S-hydroxy- 6R-[1- (R and S)hydroxypropyl]pyramido[1,2-a]purine-10-(3H)one. Similar 6-hydroxyhexyl adducts were isolated in the reaction of deoxyguanosine with t-4HN. The reactions appear to involve Michael additions of the N2 amino group of deoxyguanosine followed by cyclization at the 1-N site. This reaction mechanism is similar to that reported for deoxyguanosine adduct formation with the nonhydroxylated alpha, beta-unsaturated aldehydes crotonaldehyde and acrolein. Total adduct formations following 16-h incubations were 0.91% for t-4HH and 0.85% for t-4HN. These results demonstrate that t-4HH and t-4HN possess the ability to alkylate deoxyguanosine in vitro and suggest possible mechanisms for 4-hydroxyalkenal and pyrrolizidine alkaloid genotoxicity.