Recently diseases of cardiovascular system in children take one of the first places in Ukraine.The aim of the study – studying the morphometric features of walls and interventricular septum thickness of rat’s heart in normal conditions and after antenatal antigen impact.Materials and Methods. 144 hearts of laboratory rats were investigated. They were divided into 3 groups: I group – intact rats, ІІ – control group, that get intrauterine injection of Sodium Chloride solution 0.9 % 0.05 ml on the 18th day of antenatal development. III – the experimental group of rats, who were injected by 0.05 ml of anatoxin in a similar method. Serial sections were stained with hematoxylin and eosin.Results and Discussion. In the experimental group in newborn animals the indices of the thickness of right ventricle, left ventricle and interventricular septum are much lower than in the control. This tendency is observed until the 21st day, and on the 45th day all thickness indicators were significantly higher in experimental animals. Obtained results can be related with changes in the ratio of muscle and connective tissues by reducing of the first and increasing of the second. Wall’s thickening in experimental animals on the 45th day can be considered as a development of hypertrophic cardiomyopathy, which is one of the manifestations of UCTD in the heart.Conclusions. Intrauterine antenatal administration of the antigen may cause a disturbance in the formation of the connective tissue of the heart, which in the future may lead to decreasing of the adaptive capacity of the myocardium.