The amount of defects in DNA secondary structure recognized by nuclease S1 and their localization by sequences of various nucleotide composition were studied in relation to the age factor. It was established that a significant twofold increase in defects of DNA secondary structure was observed in hepatic cells of very old (30 months and older) intact mice only, as well as in radiation-induced acceleration of aging at the age 19 months. The prevailing localization of the defects of DNA secondary structure has been demonstrated in those sequences which were enriched with AT pairs by 3% (as compared with the average level). It was concluded that the mentioned defects of DNA secondary structure are not the cause of aging. Nevertheless they can play an essential role in the process of irreversible destruction of genome in the terminal phase of aging.