Abstract-: The study was designed to identify an index that changes shortly after irradiation and can be used for early (within 6-24 h) diagnosis of the initiation of cystitis as a complication arising during radiation therapy of patients with malignant neoplasms in the pelvic organs. Our newly developed model of radiation-induced cystitis is based on a single irradiation of the urinary bladder area in rats on a linear electron accelerator Axesse (Elekta, Sweden) with a quantum energy of 6 MeV at a dose of 25 Gy at a dose rate of 4.50 Gy/min. Early effects were estimated by a change in the total number of leukocytes and their fractions, the concentration of extracellular DNA (exDNA), and the cholesterol, gamma-glutamyl transferase, urea, and alk. phosphatase of the serum in the peripheral blood in 6 and 24 h after irradiation Within 6 h after the effect, a decrease in the total number of leukocytes, lymphocytes, and monocytes was observed (from 16.06 ± 1.83 to 10.57 ± 1.06; from 11.44 ± 1.30 to 5.84 ± 0.47; and from 0.51 ± 0.08 to 0.32 ± 0.03 (x109/L), resp.), while 24 h later, the number of granulocytes also decreased from 4.11 ± 0.57 to 2.14 ± 0.15 (x109/L). In 6 and 24 h after irradiation, there was an increase in the concentration of exDNA in the blood serum from 7.70 ± 0.55 to 11.20 ± 1.33 ng/μL normalized by 48 h after the effect. ExDNA is an index of cell death with a local effect of radiation and can be a promising early noninvasive marker of the development of radiation cystitis, which will probably be important in the choice of means for the prevention of the occurrence of radiation pathol.