Considering the preexisting influence of the process of natural aging on antioxidant enzymes activity and the level of lipid peroxidation, the age of the rats at which D-galactose (D-gal) treatment is started could strongly impact the development of D-gal induced senescence. To eval uate this, we subjected 1, 3 and 15 months old rats to D-gal treatment in parallel with having appropriate placebos (0.9 % saline). Our results showed elevated glutathione peroxidase (GPx) acti vity and no significant changes in superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR) activity or malondialdehyde (MDA) levels in relation to natural aging. In mature and aged senescent livers we observed positive correlation between increased ratio R=SOD/(GPx+CAT) and increased MDA concentration. MDA levels seemed to correlate positively with the age of the animals at which D-gal treatment had started. In the case of 3 and 15 months old rats there was D-gal induced decrease in SOD and GR activity, but this effect of the treatment was not observed in 1 month old rats. Our results imply that the changes in the antioxidant enzyme activities are not only under the influence of the D-gal overload, but also depend on the developmental stage of the rats. According to our resu lts, with regard to enzymatic antioxidant capacity and the level of lipid peroxidation, the best age for induction of senescence is somewhere after the third month., N. Hadzi-Petrushev, V. Stojkovski, D. Mitrov, M. Mladenov., and Obsahuje bibliografii