The aim of this study was to determine the pathophysiological mechanisms of postcardiopulmonary bypass (CPB) intestinal dysfunction using an in vivo canine model of extracorporeal circulation. Six dogs underwent a 90 min hypothermic CPB with continuous monitoring of mean arterial blood pressure (MAP) and mesenteric blood flow (MBF). Reactive hyperemia and vasodilator responses of the superior mesenteric artery to acetylcholine and sodium nitroprusside were determined before and after CPB. Mesenteric lactate production, glucose consumption, creatine kinase (CK) release and venous free radicals were determined. CPB induced a significant fall (p < 0.05) in MAP and MBF. After CPB, reactive hyperemia (-26 +/- 15% versus -53 +/- 2%, p < 0.05) and the response to acetylcholine (-42 +/- 9 versus -55 +/- 6%, p < 0.05) were significantly decreased. Reperfusion increased lactate production (0.8 +/- 0.09 mmol/L versus 0.4 +/- 0.18, p < 0.05) and the CK release (446 +/- 98 U/L versus 5 +/- 19 U/L, p < 0.01). Endothelial dysfunction, conversion from aerobic to anaerobic metabolism, and intestinal cell necrosis seem to be responsible for intestinal complications associated with CPB.