In one embodiment, this invention provides an antagonist of glucose-dependent insulinotropic polypeptide (GIP) consisting essentially of a 24 amino acid polypeptide corresponding to positions 7 30 of the sequence of GIP. In another embodiment, this invention provides a method of preventing and treating obesity and non-insulin dependent diabetes mellitus (Type II) in a patient comprising administering to the patient an antagonist of glucose-dependent insulinotropic polypeptide (GIP). In yet another embodiment, this invention provides a method of improving glucose tolerance in a mammal comprising administering to the mammal an antagonist of glucose-dependent insulinotropic polypeptide (GIP).