Since glucagon-like peptide-1 (7–36) amide (7–37) (GLP-1) has been found to be a potent insulinotropic hormone, it has been postulated that glucagon stimulates insulin secretion from islet beta cells through the GLP-1 receptor. We therefore examined the effects of a GLP-1 receptor antagonist, exendin (9–39) amide, on glucagon- or GLP-1-stimulated insulin release from isolated perfused rat pancreas. When infusion of 100 nmol/l exendin (9–39) amide was started 5 min before that of 1 nmol/l glucagon, the stimulation of insulin release by glucagon was similar to that found in the control situation (preinfusion with vehicle alone). By contrast, when 0.3 nmol/l GLP-1 was used in the same experimental setting, exendin (9–39) amide clearly inhibited insulin release. These results indicate that glucagon stimulates insulin release mainly through glucagon receptors but not GLP-1 receptors on islet beta cells.