The enhanced oxidative stress associated with type 2 diabetes mellitus contributes to disease pathogenesis. We previously identified plasma membrane-associated ATP-sensitive K + (K ATP ) channels of pancreatic β cells as targets for oxidants. Here, we examined the effects of genetic and pharmacologic ablation of K ATP channels on loss of mouse β cell function and viability following oxidative stress. Using mice lacking the sulfonylurea receptor type 1 (Sur1) subunit of K ATP channels, we found that, compared with insulin secretion by WT islets, insulin secretion by Sur1 -/-islets was less susceptible to oxidative stress induced by the oxidant H 2 O 2 . This was likely, at least in part, a result of the reduced ability of H 2 O 2 to hyperpolarize plasma membrane potential and reduce cytosolic free Ca 2+ concentration ([Ca 2+ ] c ) in the Sur1 -/-β cells. Remarkably, Sur1 -/-β cells were less prone to apoptosis induced by H 2 O 2 or an NO donor than WT β cells, despite an enhanced basal rate of apoptosis. This protective effect was attributed to upregulation of the antioxidant enzymes SOD, glutathione peroxidase, and catalase. Upregulation of antioxidant enzymes and reduced sensitivity of Sur1 -/-cells to H 2 O 2 -induced apoptosis were mimicked by treatment with the sulfonylureas tolbutamide and gliclazide. Enzyme upregulation and protection against oxidant-induced apoptosis were abrogated by agents lowering [Ca 2+ ] c . Sur1 -/-mice were less susceptible than WT mice to streptozotocin-induced β cell destruction and subsequent hyperglycemia and death, which suggests that loss of K ATP channel activity may protect against streptozotocin-induced diabetes in vivo.