The glucose-dependent insulinotropic polypeptide (GIP) is a 42-residue metabolic hormone that is actively being targeted for its regulatory role of glycemia and energy balance. Limited structural data of its receptor has made ligand design tedious. This study investigates the structure and function of the GIP receptor (GIPR), using a homology model based on the GLP-1 receptor. Molecular dynamics combined with in vitro mutational data were used to pinpoint residues involved in ligand binding and/or receptor activation. Significant differences in binding mode were identified for the naturally occurring agonists GIP(1-30) NH2 and GIP(1-42) compared with high potency antagonists GIP(3-30)NH2 and GIP(5-30)NH2. Residues R183(2.60), R190(2.67), and R300(5.40) are shown to be key for activation of the GIPR, and evidence suggests that a disruption of the K293(ECL2)-E362(ECL3) salt bridge by GIPR antagonists strongly reduces GIPR activation. Combinatorial use of these findings can benefit rational design of ligands targeting the GIPR.