This study was performed to investigate the effect of soy isoflavone on plasma nitrite concentration and the antioxidant enzyme activities of erythrocyte and the liver using adult male rats fed high fat diet. Seven-week old male Sprague Dawley rats were divided into three groups and fed high fat diet (15% beef tallow, 1 % cholesterol; control: IF0) or high fat diets containing isoflavone 80 ppm (IF80) or 320 ppm (IF320) for 10 weeks. Plasma nitrite concentration as a vasodilator, and antioxidant enzyme activities in erythrocytes and the liver were measured. Plasma nitrite concentration was increased by 45% and 35%, respectively, in IF80 and IF320 than in IF0 group. Erythrocyte catalase, glutathione peroxidase (GPx) and glutathione reductase (GR) activities increased by 31 %, 30% and 40% in IF320 compared to IF0 group. Especially, erythrocyte GR activity increased by 61 % in IF80 group. However, catalase activity in the liver was decreased in IF80 group. GPx and GR activities in the liver were not differ among groups. The results suggest that soy isoflavone have the protective effect against risk factors related with cardiovascular disease by improving vasodilator factor, nitrite, and antioxidant enzyme activities in blood. (Korean J Nutrition 38(2): 89～95, 2005)