Exendin (9-39) is an inverse agonist of the murine glucagon-like peptide-1 receptor. There are implications for basal intracellular cyclic adenosine 3', 5'-monophosphate levels and beta-cell glucose competence. Exendin (9-39) is a competitive inhibitor of Exendin-3 and Exendin-4. On digestive smooth muscle, exendin (9-39) behaves as an antagonist for two members of the glucagon-receptor family, GLP-1 and glicentin.