An animal model for subperiosteal tissue expansion is described. Small tissue expanders were placed bilaterally and inflated in the buccal subperiosteal pockets of the mandible. Additionally, subcutaneous expansion was performed in the breast region. Twenty-one dogs were used in three experimental groups to evaluate the optimal size of an expander for this model, the optimal inflation regimen, and the histologic changes following subperiosteal expansion. Hemicylindrical expanders, 20 mm in width, 10 mm in height, and 30 mm in length, inflated with 1 mL saline every four days, were used for this model. It was found that the periosteum was replaced by fibrous connective tissue during expansion. Capsule formation following subperiosteal expansion was much more rapid than that following subcutaneous expansion. A greater increase in vascularity was noted in the subperiosteal expansion group and the subperiosteal control group than in the subcutaneous expansion group. In the subperiosteal expansion group, a thick fibrous capsule, with minimal inflammatory response, was observed 1 week after full inflation. Leaving the fully inflated subperiosteal expanders in place more than 1 month accelerated the resorption of the underlying bone. These results support the previously reported empirical regimens of subperiosteal tissue expansion. Furthermore, the excellence of the subperiosteal expanded bed as a recipient site for onlay grafting was confirmed.