Methylating agents are widely distributed environmental carcinogens. Moreover, they are being used in cancer chemotherapy. The primary target of methylating agents is DNA, and therefore, DNA repair is the first-line barrier in defense against their toxic and carcinogenic effects. Methylating agents induce in the DNA O⁶ -methylguanine ( O ⁶ MeG) and methylations of the ring nitrogens of purines. The lesions are repaired by O ⁶ -methylguanine-DNA methyltransferase (Mgmt) and by enzymes of the base excision repair (BER) pathway, respectively. Whereas O ⁶ MeG is well established as a pre-carcinogenic lesion, little is known about the carcinogenic potency of base N -alkylation products such as N3-methyladenine and N3-methylguanine. To determine their role in cancer formation and the role of BER in cancer protection, we checked the response of mice with a targeted gene disruption of Mgmt or N -alkylpurine-DNA glycosylase ( Aag ) or both Mgmt and Aag , to azoxymethane (AOM)-induced colon carcinogenesis, using non-invasive mini-colonoscopy. We demonstrate that both Mgmt - and Aag -null mice show a higher colon cancer frequency than the wild-type. With a single low dose of AOM (3 mg/kg) Aag -null mice showed an even stronger tumor response than Mgmt -null mice. The data provide evidence that both BER initiated by Aag and O ⁶ MeG reversal by Mgmt are required for protection against alkylation-induced colon carcinogenesis. Further, the data indicate that non-repaired N -methylpurines are not only pre-toxic but also pre-carcinogenic DNA lesions.