Parathyroid hormone related peptide (PTHrP) was discovered as a factor causing humoral hypercalcemia of malignancy (HHM). In the previous reports, HHM model rats showed “dentin niche” which is known as an odontoblastic reparative response to cytotoxic agents. In the present study, PTHrP and its receptor (PTHR1) detection patterns were evaluated, during the dentin niche formation process after treatment of a representative cytotoxic agent. Rats were injected with actinomycin D or saline, 9 rats each, and 3 rats from each group were sacrificed on days 1, 3, and 7. The incisors were subjected to immunohistochemical analysis for PTHrP, PTHR1 and ED-1 (macrophage marker), as well as histopathological analysis. On day 1, single cell necrosis and concomitant vesicles that contained necrotic cellular debris were observed. The latter vesicles were positive in the PTHrP, PTHR1 and ED-1 immunostainings. On day 3, depolarization of odontoblasts was observed and hypertrophied pulpal cells were formed osteodentin which stained positively stained for both PTHrP and PTHR1. On day 7, osteodentin had progressed to form dentin niche and the cells which comprised the lesion sustained positive reactions for both PTHrP and PTHR1. These observations suggest that the PTHrP/PTHR1 axis modulates odontoblastic repair and that the modulation might initiate proteins expression in activated macrophages.