We examined whether tumor necrosis factor-α (TNF) affects the cytotoxic capacity of reactive oxygen species on rat hepatocytes in culture. Both TNF and reactive oxygen species are involved in many inflammatory events including hepatic ischemia/reperfusion injury and endotoxic shock. Synchronous treatment of hepatocytes with both TNF and H 2 O 2 demonstrated that TNF (2000 ng/ml) enhanced the cytotoxic effect of H 2 O 2 (500 μM). By contrast, pretreatment with TNF (2000 ng/ml) for 24 h followed by exposure to H 2 O 2 (1000 μM) reduced the reactive oxygen-induced cytotoxicity. We conclude that TNF increases the effects of reactive oxygen-induced cytotoxicity when exposed synchronously, whereas TNF pretreatment induces a cytoprotective effect to reactive oxygen species, presumably by up-regulation of the reduced form of glutathione levels in hepatocytes.