Objective：To investigate the effect of nebulized ketamine inhalation on the expression of nitric oxide synthase（NOS） in the lungs of asthmatic rats. Methods：Forty Brown Norway rats（10-12 weeks） were randomly assigned to five groups, with eight animals each, which were control group（C）, asthma group（A）, and ketamine pretreated groups（K1，K2，K3）. In group A，K1，K2，K3， asthma was induced in two steps：receiving subcutaneous injection of ovalbumia（OVA）1 mg and aluminum hydroxide 160 mg in 1ml of PBS, and then inhaling nebulized 1% OVA in PBS for 30 min. In group K1, K2 and K3, the sensitized rats were exposed to 12.5 mg／ml（K1 group），25 mg／ml（K2 group），50 mg／ml（K3 group） of nebulized ketamine for 30 min. Lung samples were taken and total RNA were isolated, NOS mRNA were determined with RT-PCR. The right lung was removed for microscopic examination. Results：There were acute airway inflammation changes in group A. Compared with group A, there was significantly less inflammation in the bronchial subnucosa and alveolar septum in group K1，K2 and K3. Compared with group C, the expression of iNOS mRNA was significantly higher in group A（P ＜ 0.01）. Compared with group A, the expression of iNOS was significantly less in group K1（P ＜ 0.05）, K2 （P ＜ 0.01） and Ｋ3（P ＜ 0.01）. There was no significant difference between group K1，K2 and K3 in expression of eNOS mRNA. The expression of nNOS was low in all groups. Conclusion：Inhalation of nebulized ketamine can restrain the expression of iNOS mRNA in lung in asthmatic rats and has a protective effect on airway against inflammation and tissue damage. 12.5 mg／ml nebulized ketamine appears to be enough for satisfactory clinical results.