Abstract: Objective: To research the changes and effects of nitric oxide synthase activity in atherosclerotic rabbit models. Methods: Thirty rabbits were divided into 3 groups at random: control group, atherogenic diet group, balloon-injury+ atherogenic diet group. 3 months later underwent pharmacological triggering with Chinese Russell's viper venom and histamine to make the plaque rupture. The lipid concentrations and the levels of nitric oxide (NO), NOS and hydroxy radical (OH) in the serum were obtained at different period, and pathologic changes were observed. Results: In atherogenic diet group and balloon-injury+ atherogenic diet group, the serum lipid concentrations increased markedly, but the levels of NO hadn't difference. The activity of NOS and OH levels were increased obviously after the atherogenic diet. But it reduced after pharmacological triggering and plaque rupture. Cnclusions: In earlier period of atherosclerosis, NOS activity up regulated and endothelium functions were compensated. But, When the atherosclerotic plaques were unstable, the activity of NOS descended obviously and endothelium function go to incompensation.