Some patients undergoing cardiac surgery develop excessive postoperative bleeding. Cardiopulmonary bypass causes platelet dysfunction. Several studies have documented the ability of desmopressin to reduce hemorrhage in a variety of congenital and acquired platelet disorders. In this study the investigators will investigate wether desmopressin reduces postoperative microvascular bleeding. The investigators will investigate wether desmopressin reduces platelet activation as measured by plasma concentration of neutrophil activating peptid 2 and by flow cytometry. The primary endpoint of the study will, however, be total postoperative bleeding and need for transfusions of blood components after surgery. The need for transfusions will be registered during the whole hospital stay. Patient with excessive postoperative bleeding (more than 250 ml for one hour, or more than 150ml for two hours during the first four hours) will be randomized into two groups and given either desmopressin or placebo (0,9% sodium chloride) as an intravenous infusion. Blood samples for plasma concentration measurements will be drawn before infusion of desmopressin/placebo, immediately after the infusion and 20 hours postoperatively. Postoperative bleeding will be registered for 16 hours. The need for any transfusions of blood products will be registered for the whole hospital stay.