The contribution of the pentose and Krebs cycles to glucose metabolism has been determined in the isolated goosefish islet, in vitro, using glucose-2-C-14. Hyperglycemia caused increased net glucose metabolism by both the pentose and Krebs cycles, while glucagon and tolbutamide had no significant effect on net glucose metabolism by either pathway. The rate of reduced pyridine nucleotide formation was calculated and was increased with hyperglycemia but not with tolbutamide or glucagon.