Objectives: To determine whether intestinal permeability increases during cardiac operations, and whether the degree of endotoxemia is related to this increase. Furthermore, to determine whether intestinal permeability is related to the hemodynamic state during operation and to postoperative systemic responses. Design: Prospective study. Setting: University hospital. Participants: Twenty-three male patients undergoing elective coronary artery bypass surgery. Interventions: Before surgery and during the fifth postoperative day, 100 mL of a solution containing L-rhamnose and cellobiose were administered orally. Measurements and Main Results: intestinal permeability was assessed by measuring the urinary excretion of L-rhamnose and cellobiose. Endotoxin concentrations in blood and prime fluid, hemodynamics, oxygen consumption, gas exchange, fluid balance, and the dose of vasoactive drugs were measured. Systemic responses were assessed by measuring hypermetabolism, circulatory support, and gas exchange. Intestinal permeation of cellobiose, reflecting paracellular transport, significantly increased during operation (p < 0.01), and correlated with the amount of circulating endotoxin (r2 = 0.46; p < 0.01). A high dose of ephedrine administered during operation, low baseline central venous pressure, and a less positive fluid balance during operation were associated with high intestinal permeability (r2 = 0.7; p < 0.01). Intestinal permeability was related to postoperative systemic responses (r2 = 0.49; p < 0.01). Conclusions: This study shows that during elective coronary artery bypass operations intestinal permeability between cells may increase. The degree of endotoxemia is related to this increase. Increased intestinal permeability is related to the use of ephedrine, especially during hypovolemia, and to postoperative systemic responses. Although a causative relation is not shown, these results might indicate that hypovolemia and vasoconstriction should be avoided during the operation.