Objective: The expression of eNOS and iNOS in lung tissues plays an important role in lung injury after cardiopulmonary bypass. The aim of this study is to evaluate the effect of pulmonary artery perfusion with hypothermic solution on eNOS and iNOS expression in lung tissue during CPB. Methods: 34 young children with congenital heart disease and pulmonary hypertension were divided into the control group (n=23) and protective group (n=11). The basic parameters (age, weight, data of catheterization and other assistant examination) were similar. In the control group, operation were performed using routine techniques, and in the protective group, pulmonary artery was infused with 4℃ protective solution during CPB. Immunohistochemical analysis was performed on 4-?m paraffin sections of lung tissue at end of CPB. The distribution and expression of eNOS and iNOS in pulmonary endothelium, bronchi epithelium and macrophages were studied using the immunohistochemical technique. Results: The strong iNOS-expression was showed in pulmonary endothelium (2.24?0.60) and lower iNOS-expression in bronchia epithelium (0.88?0.60) and alveolar macrophages (10.6?4.0) in protective group and lower eNOS-expression in endothelia and over iNOS-expression in alveolar macrophages and bronchia epithelia in the control group (P