Glucose cycling (GC) is increased in pancreatic islets from hyperglycemic 6-mo-old ob/ob mice. We determined whether normalization of glycemia alters islet GC and insulin release in response to glucose as well as oxidation and utilization of the glucose. Mice were treated with phlorizin in dimethyl sulfoxide (DMSO) for 10 days, which resulted in normalization of blood glucose concentrations. Controls received DMSO. The mice were fasted overnight and killed, and their islets were isolated for measurements of insulin release at 5.5 and 16.7 mM glucose and at 16.7 mM glucose plus 10 mM arginine. GC was measured by the incorporation of 3H from 3H2O into carbon 2 of glucose, glucose oxidation by the yield of 14CO2 from [U-14C]glucose, and glucose utilization by the yield of 3H2O from [5-3H]glucose. Phlorizin treatment did not alter the response of insulin to glucose and to glucose plus arginine. GC was 30% in control and phlorizin-treated animals. Glucose oxidation and utilization were also the same in both groups. In fed 10- to 12-mo-old mice exhibiting a broad range of blood glucose levels, there was no correlation between GC and either insulin release or glucose concentrations. Thus the islets of ob/ob mice exhibit an increased rate of GC regardless of glycemia. This indicates that the increased rate of GC is an important characteristic of the diabetic syndrome in these animals and not simply secondary to hyperglycemia.