Nuclear factor-&kgr;B (NF&kgr;B) plays a significant role in the coordinated transactivation of cytokine, inducible NO synthase (iNOS), and adhesion molecule genes. Although inflammation is an essential pathological feature of myocarditis, the role of NF&kgr;B in this process remains obscure. We examined the role of NF&kgr;B in the progression of rat experimental autoimmune myocarditis (EAM) and tested the hypothesis that NF&kgr;B blockade with a decoy against theciselement of NF&kgr;B can prevent the progression of EAM. Lewis rats were immunized with purified porcine cardiac myosin to establish EAM on day 0. NF&kgr;B decoy was infused into the rat coronary artery on day 0 (group NF0), 7 (group NF7), or 14 (group NF14) and harvested on day 21. Scrambled decoy was infused on day 0 (group SD0), 7 (group SD7), or 14 (group SD14) and served for control groups. The ratios of myocarditis-affected areas to the ventricular cross-sectional area of all treatment groups were significantly lower than those of the control groups (group NF0, 33±18% versus SD0, 53±14%; group NF7, 19±15% versus SD7, 50±16%; and group NF14, 34±10% versus SD14, 52±14%). Immunohistochemical and immunoblot analyses showed expression of ICAM-1, iNOS, IL-2, and TNF&agr; in myocardium of scrambled decoy groups, and this expression was effectively suppressed by NF&kgr;B decoy treatment. Thus, we found that NF&kgr;B is a key regulator in the progression of EAM and that in vivo transfection of NF&kgr;B decoy reduces the severity of EAM.