The functional characteristics of liver mitochondria and physical-chemical properties of mitochondrial membranes were studied in S rats with congenitally enhanced capacity to free radical generation in comparison to those in Wistar rats. It was shown previously that intense lipid peroxidation, numerous DNA rearrangements, protein oxidation, morbid states resembling human degenerative diseases and short life-span are characteristic of S rats. In present study we have demonstrated that in S rats at the age of 2-3 months the respiratory rate of the mitochondria in active metabolic state is lower than in Wistar rats, and so are the values for the respiratory control ratio, oxidative phosphorylation and membrane potential. By 10-12 months of age the decrease of the respiration rate and oxidative phosphorylation in S rat mitochondria become even more dramatic. These changes are associated with a decrease in the extent of dip of proteins into the membrane lipid layer or with the increase in the amount of protein aggregates. The results add to the understanding of the nature of morbid conditions developed under the effect of intensive free radical generation and to the comprehension of their role in aging.