ATP-sensitive K⁺ (K_ATP) channels are known to play important roles in various cellular functions, but the direct consequences of disruption of K_ATP channel function are largely unknown. We have generated transgenic mice expressing a dominant-negative form of the K_ATP channel subunit Kir6.2 (Kir6.2G132S, substitution of glycine with serine at position 132) in pancreatic beta cells. Kir6.2G132S transgenic mice develop hypoglycemia with hyperinsulinemia in neonates and hyperglycemia with hypoinsulinemia and decreased beta cell population in adults. K_ATP channel function is found to be impaired in the beta cells of transgenic mice with hyperglycemia. In addition, both resting membrane potential and basal calcium concentrations are shown to be significantly elevated in the beta cells of transgenic mice. We also found a high frequency of apoptotic beta cells before the appearance of hyperglycemia in the transgenic mice, suggesting that the K_ATP channel might play a significant role in beta cell survival in addition to its role in the regulation of insulin secretion.