Objective: The objective of the present study was to address the effect of mushroom Pleurotus ostreatus on the catalase (CAT) gene expression and the protein carbonyls in liver and kidney of aged (24 months old) rats. Methods: Eighteen acclimated rats were divided into 3 groups of 6 each: group I, normal young (4 months old) rats; group II, normal aged (24 months old) untreated rats; group III, normal aged rats treated with mushroom P. ostreatus extract (200 mg/kg body weight administered intraperitoneally for 30 days). On the 31st day, rats were sacrificed by decapitation; the livers and kidneys were removed, washed free of blood, blotted dry and processed immediately. Reverse transcriptase-polymerase chain reaction (RT-PCR) and spectrophotometry were utilized for the analyses of CAT gene expression and protein carbonyl content in the tissues of livers and kidneys. Results: In aged rats that had been treated with the extract of P. ostreatus (group III), the level of the transcript of CAT gene was found to be higher than that in liver (P<0.01) and kidney (P<0.05) of aged untreated (group II) rats, respectively. Treatment of aged rats with P. ostreatus extract (group III) resulted in protein carbonyl levels being significantly lower in liver (P<0.05) and kidney (P<0.01) than those observed in aged untreated (group II) rats. Conclusion: These results suggest that an extract of P. ostreatus can enhance the antioxidant enzyme (CAT) gene expression and could decrease the incidence of free radical-induced protein oxidation in aged rats, thereby protecting the occurrence of age-associated disorders that involve free radicals.