[en] A short-lived radioactive isotope ¹²³I is very convenient for clinical use. A method of obtaining Na¹²³I preparation and its experimental evaluation are given. In experiments on male albino mice of 16.5-18 g the dynamics of ¹²³I absorption, radiation effect on the thyroid gland tissue and toxicity of the preparation were studied. To study the dynamics of ¹²³I and ¹³¹I absorption the animals were subjected to a subcutaneous administration of the preparation in a dose of 2 μCi, the count was made in 1, 3, 5, 7, 9, 17 and 24 h following the administration of the isotope and was compared with the control. Other groups of animals were given 10, 25, 50 and 100 μCi of the radionuclides ¹²³I and ¹³¹I. The obtained experimental data show that ¹²³I is nontoxic, absorbed in the same amounts as ¹³¹I and causes no norphological changes in the thyroid gland tissue