The aim of the study was to investigate histological changes that occur in periodontal tissues of dogs, as a reaction to prolonged orthodontic forces. Two dogs were bonded fixed orthodontic appliance in the front region of the upper jaw. One dog received low intensity orthodontic forces and the other high intensity orthodontic forces. After two months of orthodontic forces, histological specimens were harvested, stained and examined under microscope at different magnifications. Histological examination showed the presence of tightened and compressed fibers and blood vessels, as well as the presence of extra-vascular erythrocytes. The orthodontic shifts of front teeth in dogs lead to periodontal tissue changes. The longer the action of orthodontic forces, the higher degree of fibrillary destruction in the periodontal ligament was noticed.