c-jun and jun-B are nuclear proto-oncogenes induced by growth factors such as bone morphogenetic proteins (BMPs). These gene products enhance the expression of many genes, including osteocalcin and collagen types, indicating that c-jun and jun-B play important roles in the cell differentiation process. It is also known that BMPs affect the differentiation of pulp cells to odontoblast-like cells during reparative dentinogenesis, but little is known about the transcriptional regulation of genes in cells associated with reparative dentinogenesis. In this study, we examined the expression of c-jun and jun-B in pulp cells during reparative dentinogenesis after cavity preparation of rat molars by in situ hybridization. In rat tooth germs, c-jun and jun-B were co-expressed in the odontoblastic lineage. In rat adult molars, c-jun was expressed in the odontoblast layer, but the jun-B expression was absent in all pulp cells. After cavity preparation, we found that c-jun and jun-B were coexpressed in pulp cells underneath cavities. During the early phase of reparative dentinogenesis, levels of c-jun and jun-B greatly increased in pulp cells within and around the reparative dentin matrix formed adjacent to the cavity floor. Fourteen days after cavity preparation, c-jun and jun-B were expressed only in pulp cells lining the irregular surface of the thick reparative dentin. These results suggest that c-jun and jun-B may play important roles both in physiological and in reparative dentinogenesis; in particular, the limited distribution of the jun-B expression suggests a specific role of jun-B only in cells involved with the active formation of the dentin matrix during primary and reparative dentinogenesis.