The purpose of this study is to investigate the effect of tibial lengthening on the muscle in rabbits by observing the changes of muscle in terms of the lengthening ratio of a long bone by callotasis. Seventy five growing rabbits are divided into three lengthening groups: Group I (10% lengthening), Group II (20k lengthening), and Group III(30% lengthening). The lengthening is done on the left tibiae,and the right tibiae are used as a control. Histopathologic and histomorphometric studies are done on the medial heads of gastrocnemius muscles. In i.he histopathologic study stained by hematoxylin eosin, PAS, and Masson trichrome, the mean scores of the following five subjects, each counted from 0 to 3, are analyzed for individual group between the experimental side and the control one by Wilcoxon signed rank test. The results are: 1. The atrophy of muscle fibers is significant in Group I, Group II, and Group III (p< 0.05). 2. The internalization of nuclei of muscle fibers is not significant in Group I, Group II, and Group III (p> 0.05). 3. The degeneration of muscle fibers is not significant in Group I, Group II, and Group III (p> 0.05). 4. The regeneration of muscle fibers is not significant in Group I, Group II, and Group III (p> 0.05). 5. The endomysial fibrosis of muscle fibers is significant in Group I, Group II, and Group Ill (p< 0.05). To study overall histopathologic changes, the mean of cummulated scores of the above five subiects is analyzed by Wilcoxon signed rank test. The change is not statistically signifiant in Group I (p>0.05), but there are significant changes in Group II, and Group III (p< 0.05). There are statisticaIly significant differences between groups after analyzing the mean of cummulated scores for the above five subjects by Wilcoxon rank sums test(p<0.05). In histomorphometric study with NADH-TR stain, the mean size of Type I and Type II muscle fibers is measured by an image analyzer. The statistical analysis between the experimental side and the control one for each group shows: 1. The mean size of Type I and Type II muscle fibers is decreased in every individual lengthening group with respect to the control group (paired t-test) (p< 0.05). However, this effect is not significant between the lengthening groups(Wilcoxon rank sums test) (p>0.05). 2. There is no significant change in the proportion of muscle fiber types in all lengthening groups between the experimental side and the control one (Wilcoxon signed rank test) (p>0.05). Furthermore, this effect is not significant between the lengthening groups (Wilcoxon rank sums test) (p>0.05). Following conclusions are made based on the above observations. On the tibial lengthening by callotasis in rabbits, it is shown that: 1. Changes are not observed in 10% lengthening group, but there are changes in 20% and 30% lengthening groups by histopathologic study. 2. The decrease in the mean size of Type I and Type II muscle fibers is observed when the tibiae are lengthened 1096 or more, but there is no change in the proportion of the muscle fiber types by histomorphometric study.