Objective To investigate the effects of ulinastatin on the expression of Toll-like receptor 4 (TLR4) during myocardial ischemia/reperfusion (I/R) in rats and the possible mechanism.Methods Thirty adult Sprague-Dawley rats,weighing 250-280 g,were randomly divided into 3 groups (n =10 each):sham operation group (group S),I/R group and ulinastatin group (group U).Myocardial ischemia was induced by 30 min occlusion of left anterior descending coronary artery followed by reperfusion.Ulinastatin 1 × 104 IU/kg was injected intravenously at 5 min before reperfusion in group U,while the equal volume of normal saline was given in groups S and I/R.The animals were sacrificed at 5 h of reperfusion and myocardial specimens were obtained for microscopic examination and determination of myocardial apoptosis (using TUNEL),TLR4 expression (by immuno-histochemistry) and TNF-α content (by ELIAS).The apoptotic index was calculated.Results Compared with S group,the apoptotic index,TLR4 expression and TNF-α content were significantly increased in groups I/R and U (P ＜0.05).Compared with I/R group,the apoptotic index,TLR4 expression and TNF-α content were significantly decreased (P ＜ 0.05),and the pathological changes were significantly reduced in group U.Conclusion The mechanism by which ulinastatin alleviates myocardial I/R injury is related to inhibition of TLR4 expression and reduction of inflammatory responses during early I/R in rats.