Background Coronary ischemia-reperfusion (I/R) injury causes cardiomyocyte necrosis in a multi-step process that includes an inflammatory reaction. A recent study has suggested that high-mobility group box 1 (HMGB1) is a late mediator of lethal sepsis and an early mediator of inflammation and necrosis following I/R injury. In the present study a neutralizing monoclonal antibody (mAb) for HMGB1 was used to clarify the role of HMGB1 in cardiac I/R injury. Methods and Results Rats underwent 30 min of left coronary artery occlusion followed by 60 min reperfusion. An intravenous injection of anti-HMGB1 mAb or control IgG was administered just before reperfusion. The infarct size was enlarged in the anti-HMGB1 mAb group in comparison with the control group (p< 0.05). The treatment of anti-HMGB1 mAb significantly increased the plasma troponin-T and norepinephrine (NE) content in the heart in comparison with the control (p< 0.05). Moreover, the production of dihydroxyphenylglycol was reduced in the anti-HMGB1-treated group (p< 0.05). Conclusion This study shows for the first time the effects of treatment with neutralizing anti-HMGB1 mAb on I/R injury in the rat heart. The findings support the novel view that I/R-induced HMGB1 may be an important factor in the modulation of interstitial NE.(Circ J 2008; 72: 1178-1184)