The right maxillary molars of male rats were removed under ether anaesthesia to eliminate occlusal contact with the mandibular molars. Groups of rats were killed at 1, 2, 4, 8 and 16 days after the experimental procedure. The dissected mandibles were radiographed and length of erupted portion of the tooth, height of alveolar crest and length of tooth were measured. The tensile strength of the periodontal ligament was measured by extracting the first molar from its socket. A marked and progressive decrease of the load required to extract the tooth was found in the first few days after the removal of the opposing teeth. No significant difference in extracting loads was found between the 8- and 16-day groups. Radiography showed that the teeth had erupted, that the height of alveolar crest had decreased and that the length of the root had increased during the experimental period. These changes were detectable only on the 8th or 16th day following the experimental procedure. It is suggested that the reduction in the mechanical strength of the hypofunctional rat molar periodontal ligament is closely associated with the progressive atrophy of the periodontal ligament and that normal functional activity of the teeth is important not only for maintenance of the structural integrity of the periodontal ligament but also to maintain the mechanical strength of the supporting tissues.