In order toanalyze the conditions for rapidly destructive periodontitis and to describe the early histopathological changes and bone degradation, periodontitis lesions were produced in five male Beagle dogs, 8 to 12 months of age. Cotton floss ligatures were placed around the crowns of upper and lower third and fourth premolars for periods of 4, 7, 14, 21 days. The buccal gingival tissues, being slightly inflamed initially, were clinically scored for various symptoms of inflammation. Block biopsies of premolars and the adjacent buccal periodontal tissues were taken and processed for light and electron microscopy. Depending on the ligature position in relation to the gingival margin (above, at, deep below) at the day of biopsy harvest, three groups of selected biopsies were formed, and randomly selected sections of these were used for various histometric measurements, for stereological estimation of the size and composition of the connective tissue infiltrate and the osteoclast density, and for general histopathological evaluation. The resulting data and observations implied that (1) onset and maintenance of this periodontitis lesion depend on subgingival ulceration, (2) rapid bone destruction is the result of osteoclast activity stimulated by acute inflammation, and (3) bone degradation occurs independently of the loss of collagen fiber attachment and the apical migration of the junctional epithelium.