Abstract: Objective: To study the dynamic change of nitric oxide synthase (NOS) expression of lung tissue in the development of hepatopulmonary syndrome (HPS) in rats.Methods: Twenty-four male Wistar rats were randomly divided into control group and model group.The rats in control group were fed with regular food,and the model group were copied HPS by multiple factors causing the disease.The rats were executed at the end of the fourth week,sixth week and eighth week,at least six rats at each time point in each group,the liver and lung tissue samples were collected for study.Results: The expressions of induced NOS and endothelial NOS of lung tissue were increased at the end of fourth week in the development of HPS (P<0.05),and increased gradually at the end of sixth weel and eighth week (P<0.05-P<0.01).Conclusions: NOS expression increases early in the development of HPS.