Objective: to study the morphometric parameters of the cellular structures of the thymus of rats with chronic radiation sickness. Methods:: in the laboratory, 20 males and 20 females of undigested white rats were used for research for three months, who received radiation for 20 days from the age of 2 months at a dose of 0.2 Gy (the total dose was 4.0 Gy) To simulate chronic radiation sickness, rats were irradiated with an AGAT P1 DHT apparatus with a capacity of 25,006 sGr/min (Estonia) The results obtained At 3 (60%) months of age, the thymus gland has the shape of a truncated cone (80%) with a forked base. In 3-month-old rats, the thymus mass averaged 189.22±2.35 mg. The relative weight averaged 1.68±0.53 mg. The volume of the thymus is 354.6 ± 33.1 mm2. The thickness of the thymus capsule of 3-month-old baby rats at the gate was on average 5.8±0.11, at the anterior end on average 8.1±0.19, and at the posterior end on average 6.9±0.31 microns. The depth of the trabecula was on average - 13.2 ± 0.51. The area of the thymus lobule of 3-month-old baby rats is on average – 68.4%. The wall thickness of the trabecular artery of 3-monthold rats in the proximal part averaged 18.4±0.22, and the veins averaged 16.3±0.21 microns, the inner diameter of the artery averaged 24.6±0.15, and the veins averaged 23.5±0.23 microns. Results: At 3 (60%) months of age, the thymus gland has the shape of a truncated cone (80%) with a forked base. In 3-month-old rats, the thymus mass averaged 189.22±2.35 mg. The relative weight averaged 1.68±0.53 mg. The volume of the thymus is 354.6 ± 33.1 mm2. The thickness of the thymus capsule of 3-month-old baby rats at the gate was on average 5.8±0.11, at the anterior end on average 8.1±0.19, and at the posterior end on average 6.9±0.31 microns. The diameter of the trabecula in the proximal part averaged 12.9 ±0.7, in the distal part it is on average 9.2±0.34 microns. The depth of the trabecula was on average - 13.2 ± 0.51. The area of the thymus lobule of 3-month-old baby rats is on average – 68.4%. The wall thickness of the trabecular artery of 3-month-old rats in the proximal part averaged 18.4±0.22, and the veins averaged 16.3±0.21 microns, the inner diameter of the artery averaged 24.6±0.15, and the veins averaged 23.5±0.23 microns. Conclusions: morphometry of the thymus showed that the length, perimeter and thickness vary unevenly and differently depending on age. In rats exposed to chronic radiation, the growth rate of these parameters slows down. When exposed to radiation, atrophic changes are detected in the thymus gland membrane. Large lymphocytes are found on the floor of the parapet, which undergo severe necrosis.Reticulosis and eosinophiliosis are found in the stroma.