Standard-sized holes were drilled in the tibia of rats resulting in a reproducible model which can be used to study the different stages of gap healing where practically no influence from biomechanical forces is present. Primary bone healing takes place in holes up to a diameter of 800 micrometer. In this case, no differentiated cartilage or connective tissue is formed. The medullary cavity is closed by a sealing callus. The bony splinters resulting from the operation are incorporated into the osseous framework. Revascularization results from the formation of medullary sinus which grows into the hole. The periosteal osseous depositions are supplied by the nutrient vessels, too. The centrifugal arterial bloodstream and the centripetal venous backflow are maintained under stress conditions during gap healing. Our investigations showed that after ten days drill holes in compact bone had been two-thirds filled from the medullary canal. At this point, the periosteal new bone formation had not yet reached the edge of the hole. The medullary sinus system in the medullary canal is different from the periosteal vascular network in that the former displays vessels with a regularly developed framework.