BACKGROUND: To investigate the muscular changes occurring during femoral lengthening performed in 42 rats. METHODS: The rats were randomly divided into six groups and different rates of lengthening were utilized and the rats were sacrificed at the end of the distraction. Histopathologic and histomorphometric measurements were done for all specimens. The results were analyzed statistically by Mann-Whitney U test. RESULTS: By increasing the rate of lengthening, the severity of pathologic changes such as degeneration of muscle fibers, number of vacuoles and cellular infiltration was increased. Phagocytosis of degenerated muscle fibers was observed after 14 days and cellular infiltration decreased sharply and fibrosis formed in the vacuoles and around the muscle fibers after 21 days of lengthening. Cellular infiltration was lost, the muscle had gained its normal appearance and endomysial, perimysial and epimysial fibrosis formed in the group, which were observed for 31 days. Twenty one percent of muscle fibers were atrophic but the ratio of type I to type II muscle fibers did not change. CONCLUSIONS: The findings of the current study showed that different regions with different phases of degeneration occur in the same muscle and irreversible changes in the muscle may develop even in 10 percent lengthening.