Exendin(9-39) has been shown a specific and reversible antagonist at the human GLP-1 receptor in vivo. In initial validation studies intravenous exendin(9-39) dose-dependently reduced the insulinotropic action of intravenous GLP-1. The maximal dose of 300 pmol/kg/min used in these studies was sufficient to reduce GLP-1 stimulated insulin plasma levels by about 83%. However, to quantify the contribution of incretin hormones to the incretin effect as stated above a nearly complete inhibition of the GLP-1 action is necessary.