Background. Any disturbance of the formation or organization of connective tissue such as undifferentiated connective tissue disease (UCTD) can lead to changes in extracellular matrix of the heart and deterioration of adaptive abilities in the future. The safety of using synthetic glucocorticoids in pregnant women is the subject of active discussions due to possible adverse effects of dexamethasone on the development of the fetus and, in particular, on connective tissue the heart. Objective. The purpose of this research is to study the features of distribution and dynamics of the connective tissue fibers in the ventricles of rat’s myocardium in normal conditions and after intranatal injection of dexamethasone. Methods. 144 hearts of laboratory rats were investigated. They were divided into 3 groups: the first group included intact rats, the second - control group, that get intrauterine injection of Sodium Chloride solution 0.9% 0.05 ml on the 18th day of antenatal development. The third group was formed by experimental animals, who were injected by 0.05 ml of dexametazone in a similar method. Serial sections were stained with orcein, van Gieson method and impregnation with silver carbonate by Lilly. Results. It was found that intrauterine administration of dexamethasone in rats causes a significant decrease of collagen fibers in the myocardium on 45-th day after birth comparison to control and intact groups by 30.8% and 29.1% respectively. In the structure of collagen fibers, the volume of type III fibers content at 14, 21 and 30 days were significantly higher in animals of intact and control groups than indexes of experimental group.Conclusion. Intranatal injection of dexamethasone may cause a violation in the development and lead to disorganisation of the connective tissue fibers in the ventricles of the heart. These changes may lead to development of UCTD and limitate the adaptive abilities of the heart in the future.