Ethenobases are promutagenic DNA adducts formed by some environmental carcinogens and products of endogenous lipid peroxidation. Mutation spectra in tumors induced in mice by urethane or its metabolite vinyl carbamate (Vcar) are compatible with 1,N⁶-ethenoadenine (εA) being an initiating lesion in the development of these tumors. As alkylpurine-DNA-N-glycosylase (APNG) releases εA from DNA in vitro, wild-type and APNG−/− C57Bl/6J mice were treated with Vcar and levels of εA and 3,N⁴-ethenocytosine (εC), which is not a substrate of APNG, were analyzed in liver and lung DNA. At 6 h after the last dose, levels of εA were 1.6-fold higher in DNA from APNG−/− mice and subsequently persisted at higher levels for longer than in DNA from wild-type animals, confirming that εA is released by APNG in vivo. In contrast, ∼14-fold lower levels of εC were induced by Vcar, and the kinetics of formation and persistence of εC was similar in the two mouse strains. The carcinogenicity of Vcar was compared in APNG−/− and wild-type suckling mice given a single dose of Vcar (30 or 150 nmol/g). After 1 year, only mice in the high-dose group developed hepatocellular carcinoma; however, the incidence was not higher in APNG−/− mice. Although higher levels and increased persistence of εA was observed in hepatic DNA from APNG−/− mice at 150 nmol/g Vcar, apoptosis and cell proliferation levels were similar in both strains of mice. This may explain why differences in εA formation/persistence observed here did not result in higher susceptibility of APNG−/− mice to hepatocarcinogenesis.