We compared the effects of compression-plating and open intramedullary nailing after reaming on the vascular supply to a standard fracture site, on the rate of fracture union, and on the biomechanical quality of bone after union, and on the biomechanical quality of bone after union in dogs. The proper location of a compression plate in relation to the periosteum also was investigated. Bone blood flow reached higher levels and remained elevated longer in fractures that were fixed with a rod than in those fixed with a plate. Rod-fixed fractures healed by periosteal callus, whereas plate-fixed fractures healed by endosteal callus. The fracture gained mechanical strength more slowly in the rod-fixed group than in the plate-fixed group. Subperiosteal or extraperiosteal placement of a plate revealed similar effects on bone-healing. CLINICAL RELEVANCE: Decisions to use intramedullary rods or compression plates for internal fixation of fractures should be based on knowledge of the biological effects of these procedures on blood flow at the fracture site and on the mechanical strength of the healing fracture. This study defines in quantitative terms the advantages and disadvantages of intramedullary rods compared with compression plates in the internal fixation of fractures.