Abstract d-Fructose (3.3 to 33.0 mmol/liter) caused a concentration-related increase in insulin output from rat islets exposed to d-glucose (3.3 to 7.0 mmol/liter), such an increase not being more marked in mouse islets. The fructose-induced increment in insulin release, relative to that evoked by d-glucose, was two times higher in islets exposed to d-glucose than in islets stimulated by d-mannose, 2-ketoisocaproate, or nonnutrient secretagogs. Likewise, the metabolism of d-fructose in islet cells was significantly different in the absence or presence of d-glucose. Thus, the ketose was largely channeled into the pentose phosphate pathway in glucose-deprived, but not so in glucose-stimulated, islets. In both glucose-deprived and glucose-stimulated islets, however, the magnitude of the secretory response to d-fructose was commensurate with the increase in ATP production attributable to its catabolism. These findings indicate that the metabolic fate of hexoses—and, hence, their insulinotropic capacity—is not ruled solely at the level of their phosphorylation.