The mechanical strength of the periodontium was examined by extracting the rat mandibular first molar from its socket in the dissected jaw following application of orthodontic forces. Rubber elastic bands of increasing sizes were inserted between the first and the second molars and following injections of anti-inflammatory drugs, hydrocortisone and indomethacin for 7 days. Marked decreases of the mean ultimate loads to extract the teeth were found following application of the orthodontic forces. Increases of the ultimate loads were found following injections of hydrocortisone and indomethacin. The distances pulled to reach the ultimate loads which seem to represent the extensibility and elasticity of the periodontium correlated closely with the ultimate loads. The orthodontic forces applied were estimated to be about 0.7 kilograms at the end of the experimental period. It was suggested that the mechanical strength of the periodontium is maintained by the organization and constitution of the periodontal collagen fibers, and by the attachment of fibers to the bone, which could be affected by orthodontic forces and by anti-inflammatory drugs.