Glutathione peroxidase (GPx) is one of the major cellular antioxidant enzymes in scavenging reactive oxygen species (ROS). The effects of GPx on cytotoxicity mediated by paraquat and t-butylhydroperoxide (tbOOH) in GPx overexpressing in cell lines were investigated. It was found that both paraquat and tbOOH could induce the typical features of programmed cell death (PCD) including chromatin condensation and DNA fragmentation. The GPx expressing cell clones could effciently inhibit PCD induced by tbOOH, but not by paraquat. The results suggested that GPx selectively inhibits cytotoxicity induced by ROS.