Controversy still exists as to the best method of treatment of long bone fractures. The purpose of the present study was to compare the effects on the healing bone of external fixation and metal plate fixation. In an experimental model on the rabbit tibia the stiffness of the fixation corresponded intentionally to that of the same methods in human fracture treatment. A transverse midshaft osteotomy was made on one tibia, and fixed with a steel plate (45 X 5 X 1 mm) or external mini-fixation. The animals were killed after 4, 6 or 12 weeks. The bone healing was evaluated by radiography, histology and biomechanical testing in three-point bending. There were no significant differences between plate fixation and external fixation in the pattern of bone healing, as it occurred by periosteal, intramedullary and intercortical callus after both methods. The stiffness increased more rapidly than the strength, and reached normal values after about 6 weeks. No significant differences in stiffness between the two methods were found. The strength was significantly greater after plate fixation than after external fixation at 6 weeks, while no significant differences were found at either 4 or 12 weeks. It is concluded that the speed of bone healing was more rapid after plate fixation. However, in the later stages of healing the plated bones were secondarily weakened because of the stress-protecting effect of the plate.