Glucose-dependent insulinotropic polypeptide (GIP) is a hormone secreted by the endocrine K-cells from the duodenum that stimulates glucose-induced insulin secretion. Here, we present the molecular characterization of the human pancreatic islet GIP receptor. cDNA clones for the GIP receptor were isolated from a human pancreatic islet cDNA library. They encoded two different forms of the receptor, which differed by a 27–amino acid insertion in the COOH-terminal cytoplasmic tail. The receptor protein sequence was 81% identical to that of the rat GIP receptor. When expressed in Chinese hamster lung fibroblasts, both forms of the receptor displayed high-affinity binding for GIP (180 and 600 µmol/l). GIP binding was displaced by <20% by 1 µmol/l glucagon, glucagon-like peptide (GLP-I)(7–36) amide, vasoactive intestinal peptide, and secretin. However exendin-4 and exendin-(9–39) at 1 μumol/l displaced binding by ∼70 and ∼100% at 10 µmol/l. GIP binding to both forms of the receptor induced a dose-dependent increase in intracellular cAMP levels (EC₅₀ values of 0.6–0.8 µmol/l) but no elevation of cytoplasmic calcium concentrations. Interestingly, both exendin-4 and exendin-(9–39) were antagonists of the receptor, inhibiting GIP-induced cAMP formation by up to 60% when present at a concentration of 10 μmol/l. Finally, the physical and genetic chromosomal localization of the receptor gene was determined to be on 19q13.3, close to the ApoC2 gene. These data will help study the physiology and pathophysiology of the human GIP receptor.