This study was conducted to evaluate the possible involvement of mitochondrial pathway in NaAsO2-induced apoptosis and the role of reactive oxygen species (ROS) and reduced glutathione (GSH) in the apoptotic effect in Chang human hepatocytes. The MTT assay demonstrated that sodium arsenite (NaAsO2) treatment for 24 h caused a dose-dependent decrease of cell viability. NaAsO2 treatment (0-30 microM) was also found to induce phosphatidylserine externalization, a hallmark of apoptosis; to disrupt the mitochondrial membrane potential (Deltapsi ( m )); to cause the release of cytochrome c into the cytosol, and to trigger cleavage of caspase-3 and poly (ADP-ribose) polymerase (PARP) in a dose-dependent manner. All these changes were accompanied with the enhanced generation of intracellular ROS and malondialdehyde (MDA). Increase of intracellular GSH also coincided unexpectedly. Moreover, the extracellular addition of N-acetyl-L-cysteine (NAC, 5 mM) effectively reduced the generation of ROS and MDA, and rescued the cells from NaAsO2 induced apoptosis and related alteration of mitochondria. These data suggest that the arsenic-induced cell apoptosis occurs though the mitochondrial pathway, and is mostly dependent on generation of ROS rather than GSH depletion in Chang human hepatocytes.