The activity of thymidine kinase, thymidine phosphorylase, adenosine deaminase and 5,-nucleotedase of AMP was studied in tissues, blood serum and lymphocytes of 60 healthy females and 50 females with fibrocavernous mastopathy aged 23-70. It was revealed that age-related changes in the activity of thymidine kinase in blood serum reflect the analogous changes in enzyme activity in tissues of healthy women. A dirrect correlation was established between thymidine kinase activity and age both in healthy females and those with mastopathy. A significant decrease in activity of thymidine phosphorylase was demonstrated in blood serum of patients with mastopathy aged 46-60. Demermined 4-fold increase in activity of adenosine deaminase in serum was accompanied by decreased enzyme activity in lymphocytes and decreased Lymphocyte Blast Transformation Index in the same age range. The revealed metabolic changes in DNA-precursors, metabolism in patients with mastopathy aged 46-60 might be one of the reasons of increased risk of oncological deseases in this age group.