PATHOMORPHOLOGICAL CHANGES OF THE PERIODONTAL COMPLEX IN EXPERIMENTAL BACTERIAL-IMMUNE PERIODONTITIS AND THEIR CORRECTION BY FLAVONOL. Introduction. One of the most urgent problems of modern dentistry is periodontitis; it concerns, first of all, modern treatments. Development of inflammation in the periodontal complex includes numerous links that lead to generalization and chronicity of it, loss of teeth and complications of other organs. Purpose of this research was to investigate the pathomorphological changes in the periodontal complex in experimental bacterial-immune periodontitis formation and treatment effect of the flavonol quercetin. Materials and Methods. The study was conducted with the use of non-breeding clinically healthy male rats. Experimental bacterialimmune periodontitis in experimental animals was caused by introducing complex mixture of microorganisms diluted with egg protein into periodontal tissue near the lower central incisors. Quercetin was used by intramuscular injection (100 mg/kg body weight) for 7 days. The transverse sections on a microtome were made in the thickness of 5-6 microns. The resulting preparations were stained with hematoxylin and eosin. Discussion. The histological study showed that the experimental periodontitis course was characterized by increased inflammatory response by the 14 th day of experiment, and its spread throughout the periodontium with accumulation of sufficiently large number of different cells, including neutrophils, macrophages, fibroblasts. The inflammatory reaction was accompanied with distinct structural changes. The analysis of the microscopic examination of the periodontal tissues in the animal group with experimental periodontitis showed positive dynamics of structural organization as compared with animals that were not treated during this period by the quercetin. At the same time, there was improved structuring of the gingival epithelial plate, its vertical and horizontal differentiation. The linearity of the epithelial layer became clearer and the number of dystrophic altered cells was decreased. The use of the flavonol quercetin during this period of the experimental bacterialimmune periodontitis development improved to normalization of the morphological state of periodontal tissues.