AbstractTechniques of tissue exclusion have been used previously in qualitative investigations of the vascularity of long bones, after experimental fracture; we quantified their effects on bone blood flow in rabbits. Thirty‐six adult rabbits were divided into three groups in which flow was measured, with the microsphere technique, 1 and 2 weeks after osteotomy. In Group 1, osteotomy of the tibial shaft only was done; in Group 2, osteotomy was done with exclusion of the periosteum and muscle by a silicone rubber sheath; and in Group 3, osteotomy was done with exclusion of the marrow by reaming and insertion of an intramedullary nail. All involved limbs were immobilized in a cast. In Group 1, cortical flow increased but marrow flow did not change, which suggests that the changes in cortical flow were mediated by a supply paralleling that of the marrow. In Group 2, the changes in cortical flow were abolished, which implies that this parallel supply is from the periosteum and extraosseous tissues. In Group 3, cortical flow was not significantly reduced, which demonstrates reeruitment of this periosteal and extraosseous supply. These results lend support to the hypothesis that the blood supply to the healing diaphysis is principally from the periosteum and extraosseous tissues during the early peak period of blood flo