PERK, an ER luminal eIF2alpha kinase, is essential for beta-cell function, as lack of functional PERK leads to beta-cell death and infancy onset diabetes, indicating a pro-survival role for the phosphorylation of eIF2alpha in beta-cells. PERK is activated in response to protein malfolding in the ER as part of the unfolded protein response (UPR) and it is widely believed that PERK is activated at high glucose due to an increased rate of protein synthesis, which exceeds the folding capacity of the ER. However, we demonstrate in pancreatic beta-cells that at high glucose concentration, neither PERK is activated or eIF2alpha phosphorylated. Whereas, in contrast, we show that at low glucose concentration, PERK is activated and eIF2alpha phosphorylated and that the over-expression of dominant-negative PERK inhibits this glucose dependent change in eIF2alpha phosphorylation. Indeed, by translational profiling we confirm that the eIF2alpha dependent integrated stress response is only activated at low glucose concentrations. By artificially altering cellular ATP levels, we were able to provide evidence that it is a decrease in ATP concentration activates PERK and stimulates the phosphorylation of eIF2alpha. Therefore, PERK can act as an intracellular ATP sensor in pancreatic beta-cells. The mechanism by which PERK is activated by glucose deprivation and the important physiological significance of these findings in will be discussed.