The objective of this study was to investigate the process of hard tissue formation after replantation of open-ended immature teeth in rat and morphologically type the hard tissue generated according to its degree of calcification. The pulp chamber, periapical cells and matrix were observed by light microscopy. Degree of ossification in undecalcified, polished samples was determined by contact microradiography (CMR). After tooth replantation, pulp cells and odontoblasts temporarily showed degeneration and necrosis. Thereafter, some of the surviving pulp cells showed activation, with healing and revascularization of apical tissue, proliferating to form matrix. These results together with the CMR findings showed that bone-like dentin was formed in the cervical pulp chamber, and that intracanal cementogenesis occurred in the apical area, continuing until the pulp chamber was filled with calcified tissues.