Phospholipid hydroperoxide glutathione peroxidase (GPx4) is a member of the family of selenium dependent enzymes that catalyze the reduction of cell membrane-bound phospholipid hydroperoxides in situ and thus, protects against membrane damage. The research described in this dissertation focused on (1) characterization of GPx4 activity levels in GPx4 -/- mice and cells from those mice, (2) development of a cell culture model that provides a physiologically relevant basis for evaluating the roles of GPx4 and the mechanisms of its actions, and (3) investigation of the relative susceptibility to toxicant-induced oxidative stress using this model. It was demonstrated that a GPx4-/- genotype is embryonic lethal, but that GPx4+/- mice survive and appear phenotypically normal. GPx4 activity is significantly lower in testis and liver tissue from GPx4+/- mice than GPx4+/+ mice. Cultured lung fibroblasts (LFs) from adult GPx4+/- mice had approximately 50% GPx4 activity of GPx4+/+ LFs, and were significantly more susceptible to H2O2, cadmium, and cumene hydroperoxide-induced cytotoxicity. GPx4+/- LFs had lower mitochondrial membrane potential, greater cardiolipin oxidation, and lower amounts of reduced thiols relative to GPx4 +/+ LFs, but were more resistant to further decrements in these endpoints following phospholipid hydroperoxide treatment. After treatment with CdCl 2 GPx4+/- LFs had a decreased survival response compared to GPx4+/+ LFs. Specifically, a GPx4+/- genotype was associated with increased morphological evidence of stress, decreased Akt activation, and increased caspase 3 activation. GPx4+/- cells exhibited decreased p38 and JNK activation, but the overall survival response, as measured by the ratio of survival pathway activation (Akt) to stress pathway activation (p38 and JNK), was attenuated in GPx4+/- LFs. GPx4+/- cells also exhibited diminished accumulation of high molecular weight poly-ubiquitinated proteins than GPx4 +/+ cells in response to CdCl2 treatment. The GPx4+/- mouse appears to provide a useful model for studying susceptibility to chemically induced oxidative stress, particularly where lipid peroxidation is a primary mediator of cellular injury. Furthermore, GPx4+/- LFs provide a useful model for understanding the mechanisms by which GPx4 protects against oxidative injury and mediates cellular functions in the face of chemical insult.