We were the first to demonstrate that direct application of the environmental pollutant and tobacco smoke constituent dibenzo[a,l]pyrene (DB[a,l]P) into the oral cavity of mice induced squamous cell carcinoma (SCC) in oral tissues but not in the tongue; however, the mechanisms that can account for the varied carcinogenicity remain to be determined. Furthermore, we also showed that not only dA adducts, but also dG adducts can account for the mutagenic activity of DB[a,l]P in the oral tissues in vivo. In this study, we initially focused on DB[a,l]P-induced genotoxic effects in both oral and tongue tissues. Therefore, to fully assess the contribution of these DNA adducts in the initiation stage of carcinogenesis induced by DB[a,l]P, an LC-MS/MS method to simultaneously detect and quantify DB[a,l]PDE-dG and -dA adducts was developed. Mice were orally administered with DB[a,l]P (24 nmole, 3 times per week for 5 weeks) or its fjord region diol epoxide, (±)-anti-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo[a,l]pyrene (DB[a,l]PDE, 12 nmole, single application); animals were sacrificed at 2, 7, 14, and 28 days after the last dose of carcinogen administration. Oral and tongue tissues were obtained and DNA were isolated followed by enzymatic hydrolysis. Following the development of an isotope dilution LC-MS/MS method, we successfully detected (-)-anti-cis- and (-)-anti-trans-DB[a,l]PDE-N(2)-dG, as well as (-)-anti-cis- and (-)-anti-trans-DB[a,l]PDE-N(6)-dA in oral and tongue tissues of mice treated with DB[a,l]P. Levels of (-)-anti-trans-DB[a,l]PDE-N(6)-dA were ≥2 folds higher than (-)-anti-cis-DB[a,l]PDE-N(6)-dA adduct and those of dG adducts in the oral tissues and tongue at all time points selected after the cessation of DB[a,l]P treatment. Levels of dG adducts were comparable in both tissues. Collectively, our results support that DB[a,l]P is predominantly metabolized to (-)-anti-DB[a,l]PDE, and the levels and persistence of (-)-anti-trans-DB[a,l]PDE-N(6)-dA may, in part, explain the carcinogenicity of DB[a,l]P in the oral tissues but not in the tongue.