Type 1 diabetes is thought to occur as a result of the loss of insulin-producing pancreatic β cells by an environmentally triggered autoimmune reaction. In rodent models of diabetes, streptozotocin (STZ), a genotoxic methylating agent that is targeted to the β cells, is used to trigger the initial cell death. High single doses of STZ cause extensive β-cell necrosis, while multiple low doses induce limited apoptosis, which elicits an autoimmune reaction that eliminates the remaining cells. We now show that in mice lacking the DNA repair enzyme alkylpurine-DNA-N-glycosylase (APNG), β-cell necrosis was markedly attenuated after a single dose of STZ. This is most probably due to the reduction in the frequency of base excision repair-induced strand breaks and the consequent activation of poly(ADP-ribose) polymerase (PARP), which results in catastrophic ATP depletion and cell necrosis. Indeed, PARP activity was not induced in APNG^−/− islet cells following treatment with STZ in vitro. However, 48 h after STZ treatment, there was a peak of apoptosis in the β cells of APNG^−/− mice. Apoptosis was not observed in PARP-inhibited APNG^+/+ mice, suggesting that apoptotic pathways are activated in the absence of significant numbers of DNA strand breaks. Interestingly, STZ-treated APNG^−/− mice succumbed to diabetes 8 months after treatment, in contrast to previous work with PARP inhibitors, where a high incidence of β-cell tumors was observed. In the multiple-low-dose model, STZ induced diabetes in both APNG^−/− and APNG^+/+ mice; however, the initial peak of apoptosis was 2.5-fold greater in the APNG^−/− mice. We conclude that APNG substrates are diabetogenic but by different mechanisms according to the status of APNG activity.