For the purpose of evaluating the response of the periodontal tissue in relation to the experimental tooth movement, orthodontic force was applied to 15 female rats. The animals were investigated histopathologically and histochemically by several staining methods. Findings were as follows: 1. The application of 60gm on the rat´s molar caused mesial tipping movement and undermining resorption. 2. One day after the insertion of the appliance, the osteoblast was increased in activity but not in number. 3. The osteoclastic activity was decreased as soon as removing the appliance. 4. With the P.A.S. staining, osteoclasts Stained mildly in the initial stage and stained more intensely in the last stage. Osteoclasts revealed negative reaction in the initial stage and stained mildly in the last stage. 5. Slides stained with toluidine blue revealed no conclusive variations between osteoblasts and osteoclasts.