Exendin (9-39) is a potent glucagon-like peptide 1 (GLP-1) receptor antagonist. It has also been described as an antagonist of the putative exendin receptor. Exendin (9-39) blocks the stimulatory action of GLP-1 (7- 36) amide (H- 6795) and of exendin-4 (H- 8730) on cAMP production in pancreatic acini. Moreover, exendin (9-39) was shown to be safely used to abolish the incretin effect of GLP-1 without interfering with the control of insulin secretion by circulating nutrients.