The effects of irradiation and hyperbaric oxygenation (HBO) on osteoblastic activity and angiogenesis in rabbit mandibular distraction (DO) were evaluated. Three groups were studied. The mandible of two groups received a 22.4Gy dose of irradiation. One of the irradiated groups was also given HBO, 18 times at 2.5ATA for 90min per day preoperatively. The third group was given neither radiotherapy nor HBO. Mandibular lengthening was performed unilaterally. Osteoblastic activity was assessed ex vivo by [18F]fluoride digital autoradiography. Neovascularization of distracted bone was evaluated histomorphometrically. Osteoblastic activity was higher in non-irradiated than irradiated animals. In non-irradiated rabbits, the activity was evenly distributed over the distraction area. In the irradiated groups, the activity was greater in the central third of the lengthened bone than the peripheral thirds. HBO changed the osteogenic pattern towards that of non-irradiated bone. In the non-irradiated group the number of blood vessels was 1.7-fold as compared to irradiated rabbits without HBO (P=0.0012), and the fewest number of vessels was found in irradiated rabbits without HBO. Blood vessels were more numerous in the central region than in peripheral regions in non-irradiated animals and irradiated animals with HBO, but not in irradiated rabbits without HBO therapy. It is concluded that radiotherapy disturbs distraction bone formation and neovascularization related to DO. HBO increases osteoblastic activity, but not to the level of non-irradiated bone. Angiogenic response is markedly increased by HBO.