1. The healing of the radius and tibia in dogs after compression plating of osteotomies made by a Gigli saw was studied.

2. The methods used were indian ink microangiography and terramycin labelling. The Spalteholz technique and azane colouring were used.

3. Revascularisaton of the fracture region took place both from newly formed vessels in the Haversian systems and from periosteal and endosteal vessels.

4. The fracture gap was filled at an early stage by a vascular network. Under stable conditions direct angiogenic bone formation took place around this network.

5. Rebuilding of the cortical bone in the fracture region occurred by osteoclastic activity. Groups of osteoclasts made cavities in the necrotic bone and were immediately followed by loops of vessels; behind and around the loop new bone was formed. Another form of bone absorption consisted of bundles of vessels which eroded necrotic cortical bone without new bone formation.

6. The new bone was initially oriented along the fracture gap but, by conversion into secondary osteones, it became progressively oriented longitudinally in the direction of the original bone.

7. Under stable conditions some periosteal and endosteal callus formation occurred though it was of slight importance. It regressed very soon and was seldom seen in the radiographs.