Ethylene oxide (EO) is a direct-acting SN2 alkylating agent and a rodent and probable human carcinogen. In vitro reactions of EO with calf thymus DNA in aqueous solution at neutral pH and 37 degrees C for 10 h resulted in the following 2-hydroxyethyl (HE) adducts (nmol/mg DNA): 7-HE-Gua (330), 3-HE-Ade (39), 1-HE-Ade (28), N6-HE-dAdo (6.2), 3-HE-Cyt (3.1), 3-HE-Ura (0.8) and 3-HE-dThd (2.0). Reference (marker) compounds were synthesized from reactions of EO with 2'-deoxyribonucleosides and DNA bases, isolated by paper and high performance liquid chromatography and characterized on the basis of chemical properties and UV, NMR and mass spectra. In agreement with our earlier studies with propylene oxide (PO) (Chem.-Biol. Interact., 67 (1988) 275-294) and glycidol (Cancer Biochem. Biophys., 11 (1990) 59-67), alkylation at N-3 of dCyd by EO under physiological conditions resulted in the rapid hydrolytic deamination of 3-HE-dCyd to 3-HE-dUrd. The hydroxyl group on the alkyl side chain which forms after epoxide alkylation is mechanistically involved in this rapid hydrolytic deamination. These results may provide important insights into the mechanisms of mutagenicity and carcinogenicity exhibited by EO and other SN2 aliphatic epoxides.