Aim: To examine the effects of GLP-1 and PYY_3-36, separately and in combination, on energy intake, energy expenditure, appetite sensations, glucose and fat metabolism, ghrelin and vital signs in healthy overweight men. Methods: 25 healthy, male subjects participated in this randomized, double-blinded, placebo-controlled 4-arm crossover study (BMI:29±3 kg/m2, age:33±9 years). On separate days they received a 150 min intravenous infusion of either a) 0.8pmol/kg/min PYY_3-36, b) 1.0 pmol/kg/min GLP-1, c) a+b, or d) placebo. Ad libitum energy intake was assessed during the final 30 min. Measurements of appetite sensations, energy expenditure and fat oxidation, vital signs and blood variables were collected throughout the infusion period. Results: No effect on energy intake was found after monoinfusions of PYY_3-36 (-4.2±4.8%, P=0.8) or GLP-1 (-3.0±4.5%, P=0.9). However, the co-infusion reduced energy intake compared to placebo (-30.4±6.5%, P<0.0001) and more than the sum of the monoinfusions (P<0.001), demonstrating a synergistic effect. Co-infusion slightly increased sensation of nausea (P<0.05), but this effect could not explain the effect on energy intake. A decrease in plasma ghrelin was found after all treatments compared to placebo (all P<0.05); however, infusions of GLP-1+PYY_3-36 resulted in an additional decrease compared to the monoinfusions (both P<0.01). Conclusion: Co-infusion of GLP-1 and PYY_3-36 exerted a synergistic effect on energy intake. The satiating effect of the meal was enhanced by GLP-1 and PYY_3-36 in combination compared to placebo. Co-infusion was accompanied by slightly increased nausea and a decrease in plasma ghrelin, but neither of these factors could explain the reduction in energy intake.