Aim. To evaluate morphological skin changes of the experimental animals after intradermal injection of collagenous medication. Methods. The histological and histochemical methods (staining with hematoxylin and eosin, by Van-Gieson, Mallory and Foot method) were used to study the skin of mature female rats in the area of intradermal injection of collagen preparation «Kollost» by using mesotherapy method in 2, 4, 21 and 37 days. Murine skin was compared between experimental and control groups, in which the rats had solution of dextrose (glucose) injected intradermally. Results. On days 2-4 after the injection, inflammatory reaction was weak in the form of cellular infiltration along the needle stick. Numerous macrophages resorbing fibrous elements of the medication were determined in the injection zone. On days 7-14 fibroblasts proliferation, occurrence of argyrophilic thin newly-formed collagenous fibres and significant content of glycosaminoglycanes in the granulation tissue were revealed. By the 21-st day of the experiment following the injection, the dermal plate of the skin had become more dense due to the formation of thicker collagenous bundles in regeneration zones. When impregnated by silver nitrate they became yellow-brown that was indicative of fibre maturity. On day 37, the collagen fibers of the injected preparation in the injection zone were not detected in free unsubstituted form. The skin had a typical structure. The signs of stimulation of regeneration processes were not revealed in the skin of rats from the control group following the glucose solution injection. Conclusion. The collagenous preparation did not cause any pronounced inflammatory processes in the skin following intradermal injection to mature female rats; the fibrous structures of «Kollost» are resorbed by macrophages and substituted by the collagenous fibres integrating into the tissues; the processes are accompanied by stimulation of proliferation of structural elements of the skin connective tissue.