The rate of glucose uptake and oxidation, the output of lactate, the net uptake of calcium, the release of preformed or newly synthesized insulin and, possibly to a lesser extent, the biosynthesis of proinsulin are all diminished in islets removed from fasted rats and exposed to glucose. Theophylline and dibutyryl-adenosine-3',5'-cyclic monophosphate fail to fully restore a normal secretory response to glucose, despite the fact that they increase lactate production by the islets from fasted animals. The insulinotropic action of other secretagogues, including glyceraldehyde, leucine, beta-hydroxybutyrate, and sulfonylurea is unaffected by prior fasting of the donor rats. The islets metabolism of glyceraldehyde is also unaffected by fasting. These data indicate that fasting is associated with a block in glucose metabolism in the early steps of glycolysis, prior to the triose-phosphate level, and suggest that the insulin secretory response to glucose may be closely dependent on the rate at which the hexose is metabolized by islet tissue.