Increased expression of heme oxygenase-1 (HO-1) in response to physiological stress is considered to be a protective response, which may be altered with aging. In this study, HO-1 expression was assessed following heat stress by immunoblotting of liver homogenates and isolated hepatocytes from young (6 months) and old (24 months) Fischer 344 rats and by immunohistochemistry. Livers of old rats showed higher baseline levels of HO-1, which was predominately localized to Kupffer cells. After heat stress, young animals showed a greater relative increase in hepatic HO-1, part of which was caused by increased numbers of nonparenchymal cells that were immunoreactive to HO-1. Consistent with these data, HO-1 was significantly upregulated after hyperthermia in vitro only in hepatocytes from young rats. Hence, aging alters stress-induced expression of HO-1 in a cell-specific manner, which may contribute to the diminished stress tolerance observed in older organisms.