Introduction. Vitamin D3 possesses antioxidant, anti-inflammatory, immunomodulatory, and other properties, has been shown to be effective in some autoimmune diseases, which is a prerequisite for studying its effect, when applied locally, on the clinical status and morphology of the site of injury in ulcerative colitis (UC).The aims was to study the effect of vitamin D3 in the composition of original rectal suppositories on the clinical status and morphology of the lesion of colon in experimental UC.Materials and methods. UC was modeled by two-stage administration of oxazolone. Rectal suppositories were prepared on the basis of a 10% aqueous solution of vitamin D3. The clinical status was assessed using the Disease activity index (DAI) scale. In the area of colon damage, the number of neutrophils, lymphocytes, eosinophils, histiocytes, plasmocytes and fibroblasts was examined per mm² d the tissue damage index (TDI) was calculated.Results and Discussion. In experimental UC, DAI increase an ulcerative defect is recorded in the colon, the number of neutrophils, lymphocytes, eosinophils, plasma cells, histiocytes, fibroblasts, TDI increases. The use of original rectal suppositories with vitamin D3 in experimental UC leads to a decrease in DAI, the size of the ulcer and TDI, a decrease in the infiltration of the intestinal wall by neutrophils, lymphocytes, eosinophils and plasma cells, an increase in the infiltration of histiocytes, fibroblasts.Conclusions. In experimental oxazolone-induced colitis, the clinical picture and morphology of the injury site characteristic of UC are recorded. The use of original rectal suppositories with vitamin D3 a total dose of 18,000 IU leads to a decrease in the severity of clinical signs and a decrease in the representation in the colon wall of cells involved in tissue destruction, an increase in the representation of cells mediating repair, which was reflected in a decrease in the area of the ulcer and tissue damage index , the severity of clinical signs according to DAI weakens as lymphocytes and plasma cells decrease in the area of damage to the colon, decrease in the size of the ulcer and decrease in TDI, increase in histiocytes and fibroblasts.