The efficacy of self-curing acrylic resin in the fixation of fractured mandibles was studied in eight adult mongrel dogs. Three dogs were killed at 1 month, two at 3 months, and three at 6 months after insertion of the resin. Roentgenograms taken after operation and after the dogs had been put to death demonstrated satisfactory fracture reduction and bony healing. The retentive grooves appeared larger on the postmortem roentgenograms, apparently from fibrous tissue that formed between the acrylic resin and the bone. Sections through the groove and fracture site did not show any fibrocartilage formation between the bone and the resin. Active osteoblastic activity was present at the fibrous capsule-bone interface as well as in the fracture calluses. Most sections showed acute inflammation, with a few giant cells. Whether the inflammatory component resulted from tissue breakdown over the acrylic resin or from the polymethylmethacrylate itself was not determined. There was no evidence of abnormal healing or nonunion along the fracture line and no evidence of necrotic bone. The undecalcified tetracycline-labeled sections confirmed active bone formation at the fibrous capsule-bone interface and in the fracture calluses at 1, 3, and 6 months. No mobility could be detected across the fracture site at any time after the insertion of the acrylic resin. Two of the three dogs that were killed at 1 month had incomplete bony union, as demonstrated by mobility along the fracture line seen in the resected specimens. Mucosal dehiscence over the superior acrylic fixation bar was a consistent finding. Once the resin had been either exfoliated or removed, the mucosa healed uneventfully. The insertion of methylmethacrylate did not result in blood pressure change in the dogs. The dogs maintained their preoperative weight, appetite, and jaw mobility. Although satisfactory bony healing resulted from the intraoral open reduction and fixation with self-curing acrylic resin in dogs, several factors need additional investigation before this technique can be considered for use in human subjects. The following findings in this study may contribute to healing complications in man: (1) loosening of the acrylic resin due to fibrous tissue formation between it and the bone, (2) mucosal dehiscence, (3) inflammation in and around the fibrous tissue adjacent to the acrylic resin, and (4) the necessity of bone removal to facilitate the insertion of the resin.