Pancreatic β cells secrete insulin in response to changes in the extracellular glucose. However, prolonged exposure to elevated glucose exerts toxic effects on β cells and results in β cell dysfunction and ultimately β cell death (glucose toxicity). To investigate the mechanism of how increased extracellular glucose is toxic to β cells, we used two model systems where glucose metabolism was increased in β cell lines by enhancing glucokinase (GK) activity and exposing cells to physiologically relevant increases in extracellular glucose (3.3-20 mM). Exposure of cells with enhanced GK activity to 20 mM glucose accelerated glycolysis, but reduced cellular NAD(P)H and ATP, caused accumulation of intracellular reactive oxygen species (ROS) and oxidative damage to mitochondria and DNA, and promoted apoptotic cell death. These changes required both enhanced GK activity and exposure to elevated extracellular glucose. A ROS scavenger partially prevented the toxic effects of increased glucose metabolism. These results indicate that increased glucose metabolism in β cells generates oxidative stress and impairs cell function and survival; this may be a mechanism of glucose toxicity in β cells. The level of β cell GK may also be critical in this process.