Pancreatic islets from healthy (control) and neonatally streptozocin-induced diabetic (STZ-D) rats, a model for non-insulin-dependent diabetes mellitus, were incubated with ³H₂O and 5.5 or 16.7 m M glucose. At 5.5 mM glucose, no detectable [³H]glucose was formed. At 16.7 mM, 2.2 patom · islet⁻¹ · h⁻¹ of ³H was incorporated into glucose by the control islets and 5.4 · patom · islet⁻¹ · h⁻¹ by STZ-D islets. About 75% of the ³H was bound to carbon-2 of the glucose. Glucose utilization was 35.3 pmol · islet⁻¹ · h⁻¹ by the control and 19.0 pmol · islet⁻¹ · h⁻¹ by the STZ-D islets· Therefore, 4.5% of the glucose-6-phosphate formed by the control islets and 15.7% by the STZ-D islets was dephosphorylated. This presumably occurred in the β-cells of the islets catalyzed by glucose-6-phosphatase. An increased glucose cycling, i.e., glucose → glucose-6-phosphate → glucose, in islets of STZ-D rats may contribute to the decreased insulin secretion found in these animals.