Abstract: Objective To examine the myocardial expressions of 12-lipoxygenase(12-LO),NO levels in the peripheral blood in mice with experimental autoimmune myocarditis,and analyze the relationship between their expressions and apoptosis. Methods A total of five-week-old male BALB/C mice were randomly divided into two groups: control group(C group 24) and autoimmune myocarditis group(AM group 24).AM group were immunized with porcine heart myosin(100 μg) in an equal content of CFA in a total volume of 0.2 ml and C group were injected with PBS in an equal content of CFA in a total volume of 0.2 ml at the 1th day and 7th day.Eight mice of each group were sacrificed in 14th,21th and 30th day.Expression of 12-LO and iNOS mRNA was determined by RT-PCR.Serum NO levels was measured by Nitrate reductase. The histological and ultrastructure changes were observed under light microscope and electron microscope. Results The expression of 12-LO mRNA,iNOS mRNA,NO and apoptotic index(AI) were up-regulated at the 14th day(0.68±0.28;82.99±2.64；57.51±2.17;13.20±2.54) and reached peak at the 21th day (1.53±0.27;92.74±3.99;76.44±3.01;24.97±1.54),then decreased at the 30th day(1.05±0.10;87.40±3.32;64.77±3.92;14.95±2.15).Compared with C group,These indicators were significantly higher in AM group(P＜0.05).And 12-LO mRNA expression in the AM group was significantly correlated with NO and apoptotic index(r=0.882;0.877;P＜0.05). Conclusion The expression of NO and 12-LO are found in mice with autoimmune myocarditis,and they are positively associated.12-LO involved in the pathogenesis of AM,may be affected by oxidative stress via NO.