We studied the effect of total body X-ray irradiation on the heme oxygenase-1 (HO-1) induction in the livers of young and aged rats. For this purpose, male rats were irradiated with a dose of 17 Gy. The animals were killed 5 h after irradiation. HO-1 protein expression was found to be increased two fold in aged rat liver, but lipid peroxide levels had decreased significantly where no change was observed either in inducible nitric oxide synthase protein expression or in glutathione levels. Contradictory results with respect to HO-1 and lipid peroxidation levels were obtained in the livers of young animals. These results suggest that the additional oxidative stress produced in irradiated rats may be compensated by HO-1 induction as decreased lipid peroxide levels were shown in the livers of aged animals.