Calcitonin (CT) is reported to be an effective medication for the treatment of inflammatory root resorption and to be capable of stimulating osteoblast proliferation in cell culture studies. In this study the effect of CT on the healing of osseous defects was evaluated in the mandibles of guinea pigs. After raising tissue flaps two experimental cavities were created on both sides of the corpus mandible of 33 guinea pigs. CT was applied into cavities either in hydroxypropyl methycellulose (HPMC) gel or gelatin as carrier. HPMC and gelatin alone and an empty cavity were also examined as control groups. Histopathological examinations under light microscopy were performed on weeks 1, 3, and 6. At week 1 in CT+ and CT+HPMC groups, prominent osteoblastic activity was observed when compared with control groups. At week 3 the presence of woven bone in the experimental cavity areas reflected the increased osteoblastic activity in all groups. At the end of week 6 woven bone was gradually replaced by osteogenic tissue undergoing remodelization with Haversian systems in all groups. It is suggested that the osseous healing of the experimental cavity was enhanced by CT application in early stages (i.e. at week 1). However there was no significant difference of osteogenic activity between the control and CT-treated groups at the end of weeks 3 and 6.