The author studied the effect of insulin on CCK8-stimulated secretion by the pancreas. CCK8 (0.6 nmol.kg-1) was administered to normal anaesthetized rats 30 min after the intravenous injection of insulin (10 U.kg-1), glucose (2 g.kg-1) or NaCl (controls). Pancreatic juice was collected from the intubated common bile duct. In rats given exogenous insulin, there were no statistically significant differences in total protein, amylase and trypsinogen output after CCK8 compared with the controls. In rats in which endogenous insulin secretion was stimulated with glucose, the amylase response to CCK8 was not significantly different from the control animals, but the trypsinogen response was significantly lower. The results show that insulin, in some still unknown manner, inhibits the trypsinogen secretory response to CCK8. In addition, they confirm data claiming that the synthesis and secretion of pancreatic amylase require a given critical ratio of insulin to glucose, or of insulin to the factor stimulating pancreatic secretion.