The movement of teeth during orthodontic treatment requires bone remodeling process in periodon?tal tissue. To find out the changes occuring in the cell itself, mechanical force was applied to the cultured periodontal ligament cells. Following results were obtained from measuring the changes in cyclic AMP and PGE_², ^³H thymidine incorporation amount in time lapse after application of mechanical force. 1. When mechanical force was applied to cultured PDL cells, the amount of cAMP in cells were increased significantly after 15 min. of force application, but were decreased gradually as time lapsed. 2. When mechanical force was applied to cultured PDL cells, the amount of PGE2 were increased at 20, 40, 60 min. and was significantly increased at 20 min. 3. When mechanical force was applied to cultured PDL cells, the amount of ^³H-thymidine incor?poration was some increased, but was not statistically significant.