The effects of NO inhalation in different periods on lung ischemia-reperfusion injury in rats were studied. The rat model of lung ischemia was established with occlusion of the left pulmonary hilus for 60 min. Forty rats were divided into 5 groups: group I was taken as sham operation group, group II as control group (ischemia-reperfusion injury), group III as NO preconditioning group (20 ppm NO inhalation for 30 min before ischemia), group IV (20 ppm NO inhalation for 30 min immediately after reperfusion), and group V (20 ppm NO inhalation for 30 min at 30 min after reperfusion). RT-PCR was used to assess the expression of interleukin-10 (IL-10), interleukin-1β (IL-1β), tumor necrosis factor a (TNF-α), intercellular adhesion mol. (ICAM-1) of lung tissue 3 h during reperfusion after 60 min pulmonary ischemia. The changes of water content of lung, myeloperoxidease (MPO) and malondialdedyde (MDA) of lung tissue were detected. Compared with group I, water content of lung, MDA and MPO of the control group increased significantly after reperfusion, and the expressions of IL-10, IL-lβ, TNF-α, ICAM-1 were upregulated. As compared with control group, mRNA levels of IL-10, IL-lβ, TNF-α, and ICAM-1 decreased in NO inhalation groups. Also the changes of water content of lung, MDA and MPO were ameliorated as compared with group II. But MDA and MPO of group IV were higher than those of group III and V. These results show that the best protective effect of NO inhalation on the lung ischemia reperfusion injury may be obtained by properly selecting inhalation period, avoiding production of oxygen free radicals.