This experiment was performed to study the effect of PGEZ on the bone resorption at the tooth movement by orthodontic force. The experimental animals were the Sprague-Dawley strain rats. The orthodontic force was applied by the insertion of separating clamp made of 0.014 (0.356mm) wire to the interproximal site between the 2nd and the 3rd upper right molars. In experiment I, 0.2μg, 0.4μg, 0,8μg, and 1.0μg PGE2 were locally injected at the submuc 3sa near the 2nd molar of the maxilla each. The effect was detected by the count of the number of osteoclasts appeared at the compressed surface of interradicula . bone. In experiment II, 1.0 mg/kg indomethacm (a specific inhibitor of prostaglandin synthetase) was subcutaneously injected. The effect was examined by the count of the number of cateoclasts appeared at the compressed surface of interradicular bone. The obtained results were follows; 1. The number of osteoclasts on the compressed surface of the interradicular bone increased in proportion to the increased dosage of PGE2 administered. The number of osteoclasts increased significantly at the administration of 0.8μg, and 1.0μg PGEZ in contrast to the control (P < 0.05). 2. ´ The administration of 1.0 mgJkgg indomethacin decreased the number of osteoclasts at the compressed bony surface significantly (P< 0.01).