1. A technique for measuring oxygen tension in the different tissues comprising the callus healing fractures in rabbits is described. 2. Oxygen tensions were lowest in fracture hematoma (mean of 6.3 millimeters of mercury), low in newly formed cartilage and fiber bone (mean ranged from 22.1 to 39.6 millimeters of mercury), and highest in fibrous tissue (means of 64.2 and 70.9 millimeters of mercury). 3. Oxygen tension in callus fiber bone remained low and did not rise to normal diaphyseal bone levels until reconstitution of the medullary canal was well underway. 4. The physiological role of oxygen in fracture healing is discussed.