The hormone insulin is stored in secretory granules and released from the pancreatic β-cells by exocytosis. In the consensus model of glucose-stimulated insulin secretion, ATP is generated by mitochondrial metabolism, promoting closure of ATP-sensitive potassium (K_ATP) channels, which depolarizes the plasma membrane^,. Subsequently, opening of voltage-sensitive Ca²⁺ channels increases the cytosolic Ca²⁺ concentration ([Ca²⁺]_c) which constitutes the main trigger initiating insulin exocytosis^,,. Nevertheless, the Ca²⁺ signal alone is not sufficient for sustained secretion. Furthermore, glucose elicits a secretory response under conditions of clamped, elevated [Ca²⁺]_c (refs , ). A mitochondrial messenger must therefore exist which is distinct from ATP^,. We have now identified this as glutamate. We show that glucose generates glutamate from β-cell mitochondria. A membrane-permeant glutamate analogue sensitizes the glucose-evoked secretory response, acting downstream of mitochondrial metabolism. In permeabilized cells, under conditions of fixed [Ca²⁺]_c, added glutamate directly stimulates insulin exocytosis, independently of mitochondrial function. Glutamate uptake by the secretory granules is likely to be involved, as inhibitors of vesicular glutamate transport suppress the glutamate-evoked exocytosis. These results demonstrate that glutamate acts as an intracellular messenger that couples glucose metabolism to insulin secretion.