This study is to investigate the effect of hydroxyethylpuerarin on the expression of tumor necrosis factor-alpha (TNF-alpha) and activity of nuclear factor kappa B (NF-kappaB) after middle cerebral artery occlusion (MCAO) in rats. Rats were subjected to cerebral ischemia-reperfusion injury induced by MCAO. Hydroxyethylpuerarin (10, 20, 40 mg x kg(-1), iv) was administered just 30 min before occlusion and immediately after reperfusion. After a 24 h reperfusion following 2 h of MCAO, the number of viable neurons in hippocampal CA1 region was counted by hematoxylin and eosin (HE) staining. TNF-alpha protein and its mRNA expression were examined with radioimmunoassay (RIA) and reverse transcriptasepolymerase chain reaction (RT-PCR) respectively. NF-KB activity was observed by electrophoretic mobility shift assay (EMSA), and inhibition of NF-kappaB alpha (IkappaBalpha) protein expression was evaluated by Western blotting analysis. Animals treated with hydroxyethylpuerarin had a significant increase in neuronal survival in comparison with vehicle-treated group. Hydroxyethylpuerarin significantly reduced the protein and mRNA expression of TNF-alpha following 2 h of ischemia with 24 h of reperfusion. NF-kappaB DNA binding activity and the degradation of IkappaBalpha in the cytoplasm also decreased by hydroxyethylpuerarin treatment. The protective effects of hydroxyethylpuerarin against ischemia-reperfusion injury may be mediated by decreasing the expression of TNF-alpha and the activity of NF-kappaB in rats.