Peroxisomeproliferators (PPs) cause hepatomegaly, peroxisome proliferation, and hepatocarcinogenesis in rats and mice. Conversely, hamsters are less responsive to these compounds. PPs increase peroxisomal beta-oxidation and P4504A subfamily activity, which has been hypothesized to result in oxidative stress. We hypothesized that differential modulation of glutathione-related defenses could account for the resulting difference in species susceptibility following PP administration. Accordingly, we measured glutathione S-transferase (GST), glutathione peroxidase (GPx), and glutathione reductase (GR) activities, and total glutathione (GSH) in male Sprague-Dawley rats and Syrian hamsters fed two doses of three known peroxisome proliferators [dibutylphthalate (DBP), gemfibrozil, and Wy-14,643] for 6, 34, or 90 days. In rats, decreases in GR, GST, and selenium-dependent GPx were observed following PP treatment at various time points. In hamsters, we observed higher basal levels of activities for GR, GST, and selenium-dependent GPx compared to rats. In addition, hamsters showed decreases in GR and GST activities following PP treatment. Interestingly, selenium-dependent GPx activity was increased in hamsters following treatment with Wy-14,643 and DBP. Treatment for 90 days with Wy-14,643 resulted in no change in GPx1 mRNA in rats and increased GPx1 mRNA in hamsters. Sporadic changes in total GSH and selenium-independent GPx were observed in both species. This divergence in the hydrogen peroxide detoxification ability between rats and hamsters could be a contributing factor in the proposed oxidative stress mechanism of PPs observed in responsive and nonresponsive species.