The movement of teeth during orthodontic treatment requires bone remodeling process of bone formation and bone resorption. To find out the changes occuring in the cell itself, mechanical stress was applied to the cell populations involved in the bone metabolism. Bone tissue cell populations were isolated from fetal rat calvaria and divided into OC and OB groups. Following results were obtained from measuring the changes in acid & alkaline phosphatease activity, cyclic AMP and PGE<sub>²</sub> production in time lapse after the application of mechanical stress. 1. In case of the marker enzyme of specific bone tissue cell, acid phosphatase activity was high in OC group and alkaline phosphatase activity was high in OB group. 2. After the mechanical stress was applied, acid phosphatase activity was decreased in both OC and OB groups and alkaline phosphatase activity was increase in OB group. 3. When the mechanical stress was applied for 15, 30 and 60 minutes, the production of PGE<sub>²</sub> increased in both OC and OB groups, as the time span increased. 4. When the mechanical stress was applied for 20 and 40 minutes, the production of PGE<sub>²</sub> increased in both OC and OB groups, as the time span increased.