It is intriguing that the kinetics of glucose-stimulated insulin secretion from the in situ perfused pancreas differ between the rat and the mouse. Here we confirm that insulin release in the rat is clearly biphasic, whereas in the mouse glucose essentially elicits a transient monophasic insulin release. Glucose-derived glutamate has been suggested to participate in the full development of the secretory response. The present report shows that the expression of glutamate dehydrogenase is lower in mouse than in rat or human islets, paralleling the insulin secretion profile. Addition of glutamic acid dimethyl ester mainly enhances insulin release at an intermediate glucose concentration in the rat pancreas. In the mouse preparation, glutamic acid dimethyl ester induces a sustained secretory response, both at 7.0 and 16.7 mmol/l glucose. These results are compatible with a role for glucose-derived glutamate principally in the sustained phase of nutrient-stimulated insulin secretion.