Hyperinsulinemia, a main feature of both human and animal obesity, has been demonstrated to be due to both an increased sensitivity to nutrient secretagogues and an impairment of the nervous regulation of insulin secretion. Recent studies have shown that pancreatic islet blood flow increases under conditions associated with an enhanced insulin secretion. The aim of this study was to determine whether or not changes in islet blood flow are present in hyperinsulinemic obese rats. Using the nonradioactive microsphere technique, we were able to show a significantly higher islet blood flow in obese rats either of the Zucker strain or Wistar rats after lesion of the ventromedial hypothalamus than in their respective lean controls. Subdiaphragmatic vagotomy had no significant effect on basal islet blood flow of lean rats, whereas it decreased significantly that of obese Zucker rats. Conversely, clonidine, an alpha 2-adrenergic agonist, induced a higher decrease of islet blood flow in obese than in lean Zucker rats. The injection of an intravenous bolus of glucose (375 mg/kg iv) increased significantly more islet blood flow in obese than in lean Zucker rats. It is concluded that obese rats present an increased pancreatic islet blood flow, which may result, at least in part, from exaggerated parasympathetic activity and lower than normal sympathetic activity. This could participate in the hyperinsulinemia observed in these rats.