The healing of 52 diaphyseal osteotomies in rabbit tibiae was followed up histologically from 3 to 24 weeks after rigid intramedullary nailing. The histological evaluation was made from longitudinal sections through the osteotomy area. Particular attention was paid to the fracture healing pattern. A bulky periosteal response was visible in every specimen. At 24 weeks, the external callus was always well remodeled. The osteotomy line rapidly filled with bone from 6 weeks onwards. At 24 weeks, the site of osteotomy was detectable only on the basis of slight irregularity in the cortex. The secondary gap healing seen in 19 specimens was the most common type of bone union from 6 weeks onwards. In 13 specimens, the exact type of osteonal healing was not definable, since a solid union with good cortical reconstruction was always the final outcome. Altogether, four nonunions were detected throughout the study, none of these, however, in the specimens at 24 weeks. Considerable endosteal resorption was detected at 24 weeks, at which time at least one third of the original cortex had dissappeared in all specimens. The rigid nail seems to ensure a relatively uneventful healing of the experimental osteotomies. Vast endosteal resorption and some nonunions make the use of medullary reaming in this connection doubtful.