Mechanical properties of the periodontal ligament of the incisor have been examined in sections of the mandibles of rats from 6 to 24 months of age. Mechanical measures estimated from the load-deformation and stress-strain curves did not show age-related changes, although the body and mandible weights increased gradually during the experimental period. It is suggested that qualitative and quantitative changes in the supporting tissues/unit area of the ligament are minor at a restricted region examined.