Context: The actions of both endogenous incretin hormones during a meal have not previously been characterized. Objective: Using specific receptor antagonists, we investigated the individual and combined contributions of endogenous glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) to postprandial glucose metabolism, energy expenditure, and gallbladder motility. Design: Randomized, double-blinded, placebo-controlled, crossover design. Setting: On four separate days, four liquid mixed meal tests (1894 kJ) over 270 minutes (min). Patients or Other Participants: Twelve healthy male volunteers. Interventions: Infusions of the GIP receptor antagonist GIP_(3-30)NH₂ (800 pmol/kg/min), the GLP-1 receptor antagonist exendin_(9-39)NH₂ (0-20 min: 1000 pmol/kg/min; 20-270 min: 450 pmol/kg/min), GIP_(3-30)NH₂+exendin_(9-39)NH₂, or placebo/saline. Main Outcome Measure: Baseline-subtracted area under the curve (bsAUC) of C-peptide. Results: Infusion of GIP_(3-30)NH₂+exendin_(9-39)NH₂ significantly increased plasma glucose excursions (bsAUC: 261 ± 142 mmol/L × min) during the liquid mixed meals compared with GIP_(3-30)NH₂ (180 ± 141 mmol/L × min; P = 0.048), exendin_(9-39)NH₂ (171 ± 114 mmol/L × min; P = 0.046), and placebo (116 ± 154 mmol/L × min; P = 0.015). Correspondingly, C-peptide:glucose ratios during GIP_(3-30)NH₂+exendin_(9-39)NH₂ infusion were significantly lower than during GIP_(3-30)NH₂ (P = 0.0057), exendin_(9-39)NH₂ (P = 0.0038), and placebo infusion (P = 0.014). GIP_(3-30)NH₂ resulted in significantly lower AUCs for glucagon than exendin_(9-39)NH₂ (P = 0.0417). Gallbladder ejection fraction was higher during GIP_(3-30)NH₂ compared with placebo (P = 0.004). For all interventions, energy expenditure and respiratory quotient were similar. Conclusions: Endogenous GIP and GLP-1 lower postprandial plasma glucose excursions and stimulate insulin secretion but only endogenous GIP affects gallbladder motility. The two incretin hormones potentiate each other's effects in the control of postprandial glycemia in healthy men.