Experimental heterotopic bone formation, induced by the implantation of demineralized bone matrix, is a useful model to study new bone formation and bone turnover. Alkaline phosphatase and acid phosphatase is known to be related to the mineralization and resorption of bone, respectively. The present study, therefore, was undertaken to investigate the role of various phosphatases, especially phosphoaminoacid phosphatases in the process of experimental heterotopic ossification by observing the effects of calcitonin and dexamethasone treatment. Female Sprague-Dawley rats weighing about 100g were used. Bone matrices were prepared after decalcification and defatting procedure using diaphyses of femur and tibia. Each rats were implanted six bone matrices in muscle pouches created in the upper of the abdominal wall. Calcitonin(10IU/kg) or dexamethasone(0.6mg/Kg) were administered subcutaneously from 1 week prior to implantation until sacrifice, upto 6 weeks after implantation. Five implants were pooled for determination of protein and phosphatase activities, and one implant was used for calcium determination. The results are summerized as follows. 1. The amount of calcium in the bone martix was markedly increased at second week and the highest at fifth week after implantation. Dexamethasone decreased the calcium content in the bone matrix throughout experimental periods. 2. Protein content in the bone matrix was remained constant throughout experimental period in control and calcition groups, but tended to be decreased by dexamethasone. 3. The activity of alkaline phosphatase was the highest at second week after implantation and was reduced by dexamethasone. 4. The activity of acid phosphatase was the highest at third week after implantation. Calcitonin increased and dexamethasone decreased the acid phosphatase activity in the bone matrix. 5. Phosphoserine phosphatase activity was decreased by calcitonin and dexamethasone. 6. Phosphothreonine phosphatase activity tended to be decreased by dexamethasone. 7. The activity of phosphotyrosine phosphatase was significantly decreased by dexamethasone. From these result, it is suggested that alkaline and acid phosphatase activities involved to calcification and remodeling of bone was markedly by dexamethasone treatment and the suppression of phosphotyrosine is the main factor for the inhibition of bone calcification.