Activity of Cu,Zn-superoxide dismutase (SOD) in old rats' brain was found to be decreased by 46.8% as compared to young animals. The brain concentration of Schiff bases (SB) was decreased by 13.6% in old rats, whereas concentration of diene conjugates (DC), protein peroxidation (PP), and total antioxidative activity (TAA) was the same in old as well as in young rats. The liver level of the DC and TAA was also the same. The serum level of the PP, SB, and DC was increased whereas the activity of the SOD and TAA was decreased in old rats. The findings suggest occurrence of considerable age-related changes in free radical processes as well as the organ specifics of these changes in rats.