This study investigated the effect of endodontic treatment on root resorption of autotransplanted and allotransplanted mature teeth in unmatched monkeys. The material comprised 40 mature maxillary central incisors in 20 green Vervet monkeys (Cercopithecus aethiops). Teeth were either allotransplanted or autotransplanted in pairs of monkeys matched only for the size of the root. Endodontic treatment was performed at random peroperatively with gutta percha and Kerr sealer in 10 of each of the 20 autografts and 20 allografts. The extra-alveolar period was 18 minutes in all groups. Histometric analysis, which included registration of surface and inflammatory and replacement resorption (ankylosis), was done on serial sections of the grafts 8 weeks after transplantation. Endodontic treatment almost completely reduced inflammatory resorption in both allotransplanted and autotransplanted mature teeth (p less than 0.01). Peroperative endodontics elicited a small increase in ankylosis in autografts. In allografts, it unmasked a high amount of ankylosis, presumably primarily due to alloimmune reactions against the donor periodontal ligament, leaving only a small portion of normal ligament almost exclusively located along the cervical region of the root surface. Downgrowth of periodontal pocket epithelium was limited in all groups.