Feedback regulation of insulin secretion from dog pancreatic B cells by exogenous insulin was demonstrated. Dog C-peptide immunoreactivity (CPR) concentration in the superior pancreaticoduodenal (pancreatic) vein was measured as an index of endogenous insulin secretion from B cells. Exogenous monocomponent (MC) insulin was administered into superior pancreaticoduodenal (pancreatic) artery of normal dogs which had various concentrations of blood glucose. When the blood glucose level was maintained at about 400 mg/dl, bolus injection of MC insulin (0.1 u/kg) into the pancreatic artery caused about 8.3% suppression of CPR level in the pancreatic vein. However, when the blood glucose concentration was approximately 180 mg/dl, suppression of the CPR level in the pancreatic vein was about 41%. With a fasting blood glucose concentration, exogenous MC insulin caused about 35% suppression of CPR level, but exogenous dog C-peptide (0.2 pM/kg/min) infusion did not affect the IRI concentration in the pancreatic vein.