Electrical stimulation among several factors that influence bone remodeling has been studied by many investigators with great enthusiasm in orthodontic field. The action mechanisms of Pulsed Electromagnetic Field (PEMF) are different from those of the conventional electrode application method in that PEMF induces endogenous current in the living tissues. PEMF is known to have the healing effect in nonunion of bone and osteoporosis. It is widely used in orthopaedic scopes and the possibility of using the method in clinical orthodontics is also conceivable. But the exact mechanisms by which the PEMF exerts its effects are not clearly understood. Therefore, the author wanted to see the effect of PEMF on five groups of rat calvarial cells obtained by sequential enzyme digestion method, and observed the changes in enzyme activation, collagen synthesis and ^³H-thymidine incorporation. The results were as follows: 1. Under the effect of PEMF, there were no changes in the alkaline phosphatase activity in five groups of cell populations. 2. Both the PEMF group and the PTH with PEMF group showed no changes in acid phosphatase activities and there were no differences between two experimental groups. 3. Under the effect of PEMF, there was significant increase of collagen synthesis in the group V cell population. 4. Under the effect of PEMF, there were significant increases of ^³H-thymidine incorporation in the group IV and V cell populations.