This study is to investigate the putative effect of an extract of the oyster mushroom, Pleurotus ostreatus, on reduced glutathione (GSH) and its metabolic enzymes in major organs of male albino rats. A significant (P<.05) decrease in the level of GSH was observed in liver, kidneys, heart, and brain of aged rats compared to young rats. Activities of glutathione S-transferase (GST), glutathione reductase (GR), and glucose 6-phosphate dehydrogenase (G6PDH) were significantly (P<.05) lower in the liver, kidneys, heart, and brain of aged rats. The isoform pattern of these enzymes in aged rats also revealed variations in relative concentrations, presumably due to oxidative stress during aging. Administration of the extract of P. ostreatus to aged rats resulted in a significant (P<.05) increase in the levels of GSH and elevated activities of GST, GR, and G6PDH in liver, kidney, heart, and brain tissues. An increased staining intensity of isoforms of GST and G6PDH was also noted in aged rats that had been treated with the mushroom extract compared to aged untreated rats. The results of this study may suggest that an extract of P. ostreatus, a potential antioxidant, can prevent the oxidation of GSH and protect its related enzymes during aging.