Exocyclic adducts are unique DNA modifications resulting from binding at two sites of bases that normally are involved in hydrogen-bonding for maintaining the double-helical structure of DNA. These adducts have been shown to be formed in rodents upon exposure to carcinogens. Using a sensitive 32P-postlabeling method combined with high performance liquid chromatography, we obtained evidence that 1,N2-propanodeoxyguanosine adducts of acrolein (AdG) and crotonaldehyde (CdG) are present in the liver DNA of humans and rodents without carcinogen treatment. The identities of these adducts were verified by cochromatography with the synthetic adduct standards. Further proof of identities was obtained by conversion mediated by nuclease P1 of the labeled AdG and CdG 3',5'-bisphosphates to their corresponding 5'-monophosphates. This treatment converted the in vivo adducts into products that again cochromatographed in a characteristic pattern with the synthetic 5'-monophosphates of AdG and CdG. Using this assay, we also demonstrated the in vivo stereoselective formation of one of the AdG isomers. The estimated total levels of modification were 1.0-1.7, 0.2-1.0, and 0.3-2.0 adducts in 10(6) guanine bases in the liver DNA of mice, rats, and humans, respectively. The detection of these adducts in relatively high levels without carcinogen treatment suggests that the endogenous factors such as lipid peroxidation may be important for their formation. This study provides evidence for the presence of acrolein- and crotonaldehyde-derived exocyclic adducts as common lesions in the liver DNA of rodents and humans.