The progression of aging affects various blood parameters, which can weaken the viability of the body. Clarification of various aspects of this process requires additional observations, especially with the use of animals. The study was conducted on 43 healthy outbred male rats aged 24 months. Also, 32 healthy outbred male rats 3 months old, which formed the control group, were taken under observation. The examined 24-month-old rats showed standard aging phenomena. In their erythrocytes, there was an increase in lipid peroxidation caused by a weakening of the activity of their antioxidant defense. In the blood of two-year-old rats, there was a decrease in the level of erythrocyte-discocytes. This was accompanied in them by an excess content of erythrocytes in the blood, which had a changed shape to varying degrees. In rats of the older group, erythrocyte aggregation was increased, as indicated by an increase in the numberof erythrocytes that entered the aggregates in the blood and an increase in the number of most erythrocyte aggregates with a decrease in the level of free erythrocytes. For two-year-old rats, low content of erythrocytes-disconcytes in the blood and an excess number of erythrocytes with a modified shape, as well as an increase in their aggregation, are characteristics. An important reason for these changes is the weakening of the antioxidant protection of erythrocytes, leading to a pronounced activation of lipid peroxidation in them.