In skeletal muscles, oxygen free radicals generated during ischemia -reperfusion are known as inducers that cause cellular injury and apoptosis and contribute to the pathogenensis of reperfusion injury. Ischemia -reperfusion for 2 hours may cause reversible changes, while prolonged ischemia -reperfusion causes irreversible changes. Following ischemia -reperfusion, diverse signals are transduced to induce a variety of gene expression. Ischemic preconditioning, defined as brief episodes of ischemia and reperfusion, is known to provide protection from the consequences of prolonged ischemia followed by reperfusion. NF -kappa B is a transcription factor that activated during ischemic preconditioning and ischemia -reperfusion. It initiates inflammation through inducing transcription of proinflammatory, procoagulant and vasoactive gene, while mediates the expression of cytoprotective proteins that block apoptosis or inhibit inflammation. The present study was performed to study the change of NF -kappa B immunoreacitvity in rat anterior tibialis and soleus muscles in response to ischemia -reperfusion and preconditioning. Experimental animals, Sprague -Dawley rats (250 ~300 g), were divided into 6 groups; 1) control, 2) ischemic preconditioning, 3) 2 hours of ischemia, 4) 4 hours of ischemia, 5) 2 hours of ischemia after ischemic preconditioning, 6) 4 hours ischemia after ischemic preconditioning. For ischemic preconditioning, left common iliac artery was occluded three times for 5 minutes followed by 5 minutes of reperfusion using vascular clamp. Ischemia was done by occlusion of the same artery for 2 or 4 hours. The specimens of tibialis anterior and soleus muscles were obtained 0, 1, 3, 6, and 24 hours after onset of reperfusion. The specimens were paraffin sectioned at 6 micrometer and NF -kappa B expression was examined using immunohistochemical methods. The results obtained were as follows : 1. In normal control group, immunoreactivity of NF -kappa B was moderate to strong in tibialis anterior muscles and weak in soleus muscles. 2. In tibialis anterior, immunoreactivity of NF -kappa B was decreased in 2 and 4 hours of ischemia comparede with normal control group. In soleus muscle, immunoreactivity of NF -kappa B was decreased in 2 hours of ischemia but it was comparable to that of normal control group in 4 hours of ischemia. 3. Ischemia for 4 hours induced more remarkable change in NF -kappa B immunoreactivity than that for 2 hours. 4. After ischemic preconditioning, changes in NF -kappa B immunoreactivity after 2 and 4 hours of ischemia were decreased compared with normal control group. 5. In ischemia for 2 and 4 hours, changes in NF -kappa B immunoreactivity of tibialis anterior muscles were more severe than that of soleus muscles. These results suggest that in the skeletal muscle, changes in NF -kappa B immunoreactivity of 4 hours of ischemia were more remarkable than that of 2 hours ischemia, and changes in NF -kappa B of tibialis anterior muscles were more remarkable than that of soleus muscles. Ischemic preconditiong attenuated the alteration of the NF -kappa B immunoreactivity induced by ischemia -reperfusion in the muscles.