Hydroxyapatite (HAP) is regarded as a useful material for grafting in procedure aimed at bone replacement. The purpose of this investigation, as a basic experiment, was to demonstrate bone formation in the dental extraction sockets during orthodontic movement of the neighboring teeth in the initial wound healing phase, and to clarify effects of indirect forces through alveolar bone on repairing bone in extraction sockets. Secondary we examined changes in the bone formation in the sockets during the tooth movement which were plugged with HAP granules and compared with the repairing bone without HAP. For this study seven dogs were used and were scarificed according to a schedule yielding observation periods up to 6 weeks. The animals were injected continuously with tetracycline during the experimental period. Undecalcified mesio-distal specimens 65-100 microns of thickness were prepared at first for microradiogram and then subjected to fluorescence microscopy. The observations indicated that the bone formation seemed to be accelerated with HAP implantation, however the alveolar trabeculae around HAP were developed during the tooth movement less than the control. The tooth movement in implanted sites was faster than in the sites without HAP implantation. The decreased response on alveolar bone formation, incident to experimental tooth movement into extraction sockets, was found in the pressure area, mesial part of socketswall. It might be induced by excessive stress during the tooth movement, amplified by the change of bone repairing due to the HAP implantation.