The development of the obesity-associated hyperglycemic syndrome in ob/ob mice, genetically determined, was observed over time by a combined functional and structural study of pancreatic islets. Islet areas increased with advancing age in ob/ob mice from 2 times at 1 month to 30 times at 6 months of age the size of lean mouse islets. Islet areas apparently increased more than pancreatic insulin content in ob/ob mice. Glucose and insulin tolerance tests were performed to study in vivo responses to glucose and insulin, respectively, in 1-, 3-, and 6-month-old mice. With ob/ob mice, glucose tolerance tests revealed more elevation of plasma glucose than in lean mice, the lean mice revealing more elevated plasma insulin than the obese mice. In insulin tolerance tests, lean mice presented marked hypoglycemia, whereas ob/ob mice revealed slightly higher plasma glucose at 1 month of age but three to four times higher amounts than that of lean mice at 6 months of age. Thus, increasing insulin resistance in ob/ob mice older than 3 months is associated with progressively increasing islet area, which contains proportionally less insulin than that of lean mouse pancreas. The data suggest that insulin resistance in ob/ob mice progressively develops up to 6 months of age and that marked islet hyperplasia is likely in response to sustained hyperglycemia, leading to hyperinsulinemia and eventual marked obesity.