A 474 microns thick latex elastic band was inserted between the mandibular first and second molars to cause tooth movement. The mechanical strength of the periodontium was measured by extracting the first molar from the socket in the dissected jaw. From the 1st to the 4th day, the ultimate extraction loads decreased markedly, while the inter-dental spaces remained about 300 microns. About 5 min after the elastic insertion, the ultimate load was not significantly changed and the inter-dental space was 61 microns. Restoration of the mechanical strength of the periodontium occurred gradually after the removal of the band. On the 4th day, recovery was complete. Restoration of the original inter-dental space was most marked in the initial 24 h. The forces exerted on the teeth by the bands were of the order of several tens of newtons initially and became reduced to a few newtons after the tooth movements. It is suggested that decreases in the ultimate extraction loads were caused by changes in constitution of the periodontal collagen, by the disorganization of the periodontium and by the loosening of the attachment of the periodontal fibres to the bone.