Effects of chemical ablation of the GIP and GLP-1 receptors on metabolic aspects of obesity-diabetes were investigated using the stable receptor antagonists (Pro3)GIP and exendin(9-39)amide. Ob/ob mice received a daily i.p. injection of saline vehicle, (Pro3)GIP, exendin(9-39)amide or a combination of both peptides over a 14-day period. Non-fasting plasma glucose levels were significantly (p<0.05) lower in (Pro3)GIP-treated mice compared to control mice after just 9 days of treatment. (Pro3)GIP-treated mice also displayed significantly lower plasma glucose concentrations in response to feeding and intraperitoneal administration of either glucose or insulin (p<0.05 to p<0.001). The (Pro3)GIP-treated group also exhibited significantly (p<0.05) reduced pancreatic insulin content. Acute administration of exendin(9-39)amide immediately prior to re-feeding completely annulled the beneficial effects of sub-chronic (Pro3)GIP treatment, but non-fasting concentrations of active GLP-1 were unchanged. Combined sub-chronic administration of (Pro3GIP) with exendin(9-39)amide revealed no beneficial effects. Similarly, daily administration of exendin(9-39)amide alone had no significant effects on any of the metabolic parameters measured. These studies highlight an important role for GIP in obesity-related forms of diabetes, suggesting the possible involvement of GLP-1 in the beneficial actions of GIP receptor antagonism.