Proplast I, Proplast II and freeze-dried homogenous cartilage (1 x 2 x 5 mm) were implanted under and over the rabbit mandibular periosteum. Histological observations and contact microradiography (CMR) were performed to investigate the healing process in the surgical sites from 1 to 24 weeks after the operation. In addition, microangiography (MA) was carried out to investigate changes of vascularization in the groups in which Proplast implants were made. Results 1. Proplast implantation group 1) In the supraperiosteal placement group, CMR revealed no new bone formation around or inside the Proplast and no morphological changes of the host bone in contact with the Proplast. In the subperiosteal placement group, from the early postoperative stage, new bone formation occurred from the host bone beside the Proplast. New bone penetrated the Proplast from the new bone region and from the host bone in contact with the Proplast. In 6 months after the operation, Almost all Proplast pores were filled with new bone. From a comparatively early postoperative stage, resorption fossae appeared in the host bone surface. In addition, Haversian canals inside the host bone enlarged and the host bone cortex gradually grew thinner. No differences in new bone volume or host bone changes were noticed between the Proplast I and Proplast II groups. 2) In the supraperiosteal placement group, granulation tissue proliferation with slight round cell infiltration was observed around and inside the Proplast at an early stage. In 2 weeks after the operation, blood clots disappeared from the Proplast, in the center of which granulation tissue proliferated. Thereafter, granulation tissue was gradually transformed into fibrous tissue, although no new bone was observed in any region. Change almost never occurred in the host bone cortex, but slight swelling occurred in the periosteum in contact with the Proplast after 1 week. 3) In the subperiosteal placement group, swelling and slight round cell infiltration were observed in the periosteum covering the Proplast at an early postoperative stage. But, after 4 months, the periosteum had returned to normal condition. Beside and inside the Proplast, rapid granulation tissue proliferation and new bone formation were observed. New bone formation was observed beside the host bone, but not beside the periosteum. For a comparatively long time, resorbed fossae in the host bone surface and enlarged Haversian canals persisted. Host bone width gradually decreased.(ABSTRACT TRUNCATED AT 400 WORDS)