4-Oxo-2-hexenal (4-OHE), which forms a 2'-deoxyguanosine (dG) adduct in a model lipid peroxidation system, is mutagenic in the Ames test. It is generated by the oxidation of omega-3 fatty acids and is commonly found in dietary fats, such as fish oil, perilla oil, rapeseed oil, and soybean oil. 4-OHE also forms adducts with 2'-deoxyadenosine (dA), 2'-deoxycytidine (dC), and 5-methyl-2'-deoxycytidine (5-Me-dC) in DNA. In this study, we characterized the structures of these adducts in detail. We measured the amounts of 4-OHE-DNA adducts in mouse organs by LC/MS/MS, after 4-OHE was orally administered to mice. The 4-OHE-dA, 4-OHE-dC, 4-OHE-dG, and 4-OHE-5-Me-dC adducts were detected in stomach and intestinal DNA in the range of 0.25-43.71/10(8) bases. After the 4-OHE administration, the amounts of these DNA adducts decreased gradually over 7 days. We also detected 4-OHE-dC in human lung DNA, in the range of 2.6-5.9/10(9) bases. No difference in the 4-OHE adduct levels was detected between smokers and nonsmokers. Our results suggest that 4-OHE-DNA adducts are formed by endogenous as well as environmental lipid peroxides.