AIM: To investigate the tight junction protein expressions of intestinal mucosa in an experimental model of cardiopulmonary bypass (CPB) in rats. METHODS: Thirty anesthetized rats were randomly divided into two groups: Group S (n=10) served as sham operation and group C (n = 20) served as CPB which underwent CPB for 1 h. Expression of occludin and zonula occludens-1 (ZO-1) were determined by West blotting and immunotochemistry, respectively. Plasma levels of diamine oxidase (DAO) and d-lactate were determined using an enzymatic spectrophotometry.RESULTS: Immunohistochemical localization of occludin and ZO-1 showed disruption of the tight junctions in enterocytes lining villi at the end of CPB and 2 h after CPB. The intensities of the occludin and ZO-1 at the end of CPB were lower than those of control group (76.4%±22.5% vs 96.5%±28.5% and 62.4%±10.1% vs 85.5%±25.6%, P <0.05) and were further lower at 2 h after CPB (50.5% ±10.5% and 45.3%±9.5%, P <0.05). Plasma d-lactate and DAO levels increased significantly (8.688 ±0.704 vs 5.745±0.364 and 0.898±0.062 vs 0.562±0.035, P<0.05) at the end of CPB compared with control group and were significantly higher at 2 h after CPB than those at the end of CPB (9.377 ±0.769 and 1.038±0.252, P <0.05). There were significant negative correlations between occludin or ZO-1 expression and DAO (r2= 0.5629,r2= 0.5424, P <0.05) or d-lactate levels (r2 = 0.6512,r2 = 0.7073, P <0.05) both at the end of CPB and 2 h after CPB.CONCLUSION: CPB markedly down-regulates the expression of occludin and ZO-1 proteins in intestinal mucosa of rats. The close correlation between expression of tight junctions (TJs) and plasma levels of DAO or d-lactate supports the hypothesis that intestinal permeability increases during and after CPB because of decreases in the expressions of TJs.