Abstract: Objective To study the effects of Dexmedetomidine on stress response and intestinal mucosal barrier function in patients undergoing heart valve replacement under cardiopulmonary bypass (CPB). Methods Forty patients with rheumatic heart disease who were scheduled for cardiac valve replacement at the First Affiliated Hospital of Bengbu Medical College from January, 2016 to December, 2016 were randomly divided into control group (group C) and Dexmedetomidine group (group D) according to a random number table, with 20 cases in each group. Before the induction of anesthesia, the patients in group D were intravenously infused with 1 μg/kg of Dexmedetomidine for 10 min followed by continuous infusion at the rate of 0.3 μg/(kg·h) until the end of operation. While the patients in group C were given intravenously with the equal volume of normal saline. The methods of induction and maintenance of general anesthesia are the same in both two groups. Arterial blood samples were collected before administration (T₀) and just the initiation of CPB (T₁), 30 min after CPB (T₂), at the end of CPB (T₃), 4 h (T₄) and 24 h (T₅) after surgery to measure concentrations of cortisol and glucose, and the activity of diamine oxidase (DAO). Results Compared with T₀, concentrations of cortisol and blood glucose on T₁-T₅, and the activity of DAO on T₂-T₅ were significantly increased (P<0.05) in both two groups. Compared with group C, the concentration of cortisol on T₁-T₅ and blood glucose on T₁-T₄, and the activity of DAO on T₂-T₅ were significantly decreased (P<0.05) in group D. Conclusion Dexmedetomidine is capable to improve the intestinal mucosal barrier function in patients undergoing heart valve replacement under CPB, which may be related to the reduction of stress response.