The inhibition of cholecystokinin (CCK)-induced pancreatic secretion by glucagon was studied in four dogs with chronic pancreatic fistulas and open gastric fistulas. After establishing a constant level of secretion by intravenous infusion of CCK at a constant rate (doses: 3, 6, 12 and 24 U/kg/h) glucagon was given by constant intravenous infusion for 1 h in constant doses (6, 12, 25 and 50 μg/kg/h). From the dose response curves to CCK alone and CCK plus 25 μ/kg/h glucagon, the Michaelis-Menten analysis was typical of competitive inhibition, i.e., the calculated maximal response was similar and the dose required for half maximal response was different. Glucagon also inhibited pancreatic secretion elicited by intraduodenal instillation of a solution of L-tryptophan and L -leucine. It was concluded that glucagon inhibits the CCK-stimulated pancreatic secretion in the dog and that this inhibition is competitive.