Abstract: Objective: To study the effects of losartan on lung injury during cardiopulmonary bypass(CPB) in dogs,and its mechanism.Methods: Twelve adult healthy mongrel dogs,weighing(14.25±1.60) kg,were randomly divided into the control group(group C,n=6) and losartan group(group L,n=6).The CPB model of dog was established.The group L and C were intravenously injected with 1 mg/kg of losartan and 0.9% sodium chloride at 30 min before CPB,respectively.The plasma levels of angiotensinⅡ(AngⅡ) and malondialdehyde(MDA) in two groups were measured before injecting(T₁),at 45 min after aortic cross-clamping(T₂),and at 30 min(T₃) and 60 min(T₄) after aortic unclamping.The oxygenation index(OI) and respiratory index(RI) were calculated at T₁ and T₄,and their lung tissue structures were observed under optical microscope.Results: Compared with T₁,the plasma levels of AngⅡ and MDA in two groups at T_2-4 increased significantly(P<0.01),and the OI and RI at T₄ in two groups decreased and increased,respectively(P<0.05-P<0.01).The differences of the plasma levels of AngⅡ in all time points between Group L and C were not statistical significance(P>0.05).At T₄,the level of MDA in group L was significantly lower than that in group C(P<0.01).The differences of the OI and RI at T₄ between two groups were not statistical significance(P>0.05),and the lung injury in group L was lighter than that in group C at T₄.Conclusions: Losartan can block the role of AngⅡ,and restrain oxidative stress,which can prevent the lung injury caused by CPB.