Experiments in rabbits were conducted to test the clinical propositions that a) the risk of nonunion in certain long bone fractures can be sharply reduced by open reduction and internal fixation, and b) that there is an optimum critical period following trauma in which this procedure should be carried out to facilitate healing. Bilateral standard open fractures of rabbit radii were internally fixed by intramedullary K-wires immediately or at 5, 10, and 17 days after trauma. These bones were subjected to stress and histometric analyses at 4-56 days after the initial fractures. The observations suggested that the pace of fracture healing could be optimized by time-delay surgery. The most exuberant and strongest calluses developed when the bones were fixed 10 days after fracture, and the rate of healing was improved. Delay times of 5 or 17 days did not provide better healing than that achieved by an immediate operation. Callus quality, in terms of the proportion of fiber bone, lamellar bone, and cartilage, was similar in all groups. The tensile strengths of the bones during callus formation were highly correlated with the ratio of callus/cortical bone areas (p less than 0.01).