We have demonstrated that in vitro brief ischemia activates nuclear factor (NF)-κB in rat myocardium. We report in vivo ischemia-reperfusion (I/R)-induced NF-κB activation, IκB kinase -β (IKKβ) activity, and IκBα phosphorylation and degradation in rat myocardium. Rat hearts were subjected to occlusion of the coronary artery for up to 45 min or occlusion for 15 min followed by reperfusion for up to 3 h. Cytoplasmic and nuclear proteins were isolated from ischemic and nonischemic areas of each heart. NF-κB activation was increased in the ischemic area (680%) after 10 min of ischemia and in the nonischemic area (350%) after 15 min of ischemia and remained elevated during prolonged ischemia and reperfusion. IKKβ activity was markedly increased in ischemic (1,800%) and nonischemic (860%) areas, and phosphorylated IκBα levels were significantly elevated in ischemic (180%) and nonischemic (280%) areas at 5 min of ischemia and further increased after reperfusion. IκBα levels were decreased in the ischemic (45%) and nonischemic (36%) areas after 10 min of ischemia and remained low in the ischemic area during prolonged ischemia and reperfusion. The results suggest that in vivo I/R rapidly induces IKKβ activity and increases IκBα phosphorylation and degradation, resulting in NF-κB activation in the myocardium.