It has long been known that the central nervous system (CNS) directly affects pancreatic insulin release. This study was undertaken to determine the effect of the CNS on pancreatic insulin release in three-month-old female lean (Fa/Fa) and hyperinsulinemic obese (fa/fa) Zucker rats. Chloral hydrate (400 mg/kg) was used as the anesthetic agent. The in situ brain-pancreas perfusion model with intact pancreatic innervation was used in this investigation. The study measured insulin secretion in response to a 60-minute glucose stimulus (200 mg/dl). CNS-intact and CNS-functionally ablated obese and lean rats were used. During the 60-minute perfusion period significantly more insulin was released by pancreata from obese rats compared to those from lean rats. In lean rats, about twice as much insulin was released by pancreata from CNS-ablated rats than from CNS-intact rats (P < 0.05), demonstrating a CNS tonic inhibition of insulin secretion. In obese rats, there was no significant difference in insulin released by the pancreata of the CNS-intact and CNS-ablated rats. To determine if there was a masking effect of predominant PNS activity over the SNS in the CNS-intact obese rats, bilateral vagotomy was performed in a group of otherwise CNS-intact obese rats prior to the onset of perfusion. Tonic inhibition was still not observed in the CNS-vagotomized obese rats. In conclusion, hypersecretion of insulin in obese rats is partially due to diminished tonic sympathetic nervous system inhibition of insulin release. These results provide additional evidence regarding abnormal CNS control of insulin secretion in obese Zucker rats.