Most studies of limb lengthening have concentrated on the osteotomy. In the present study, the response of soft tissue (muscle, artery, nerve) to different length, rates or rhythms of distraction have been investigated to define the nature of any damage and to see whether new muscle is created. The purposes of the this study are to evaluate the optimum condition for soft tissue during limb lengthening and to study the effect of different rates & rhythms of tibial lengthening on the soft tissue in rabbits hy observing the changes of muscle, artery and nerve. We lengthened the right tihiae of fifty-four growing New Zealand white rabbits by callotasis. The left tibiae were used as control. The rabbits were divided into three different rates & rhythms groups: Group 1 (increments of 0.5mm /day, divided 2 times/day), Group II (increments of 0.5mm/day, divided 3 times/day), Group III (increments of 10mm/day, divided 2 times/day). Each Group was subdivided into three lengthening groups: 1 ( 10% lengthening), 2 (20% lengthening), 3 (30% lengthening). At the end of lengthening, histopathologic & histomorphometric studies were done on the medial heads of gastrocnemius muscles, the posterior tibial artery and the posterior tibial nerve. In the histopathological study, these were stained by hematoxylin eosin, PAS and observed by light microscopy. Electron microscopic examination was done in all samples. In light microscopic findings, the sum of scores of the following five suhjects, each counted from 0 to 3, were analysed for individual groups between the experimental side and the control by the Mann-Whitney test and the kruskal-Wallis test. The following conclusions were made hased on the above observations; 1. There was no significant difference between groups I, II and III, but there was a significant dif-ference between the 10%, 20% and 30% lengthening groups by histopathologic study. And the predominant responses of muscle to the lengthening were atrophy and endomysial fibrosis. 2. There was no change in the proportion of the muscle fiber types by histomophomeric study. 3.ln the initial phase, the muscle adaptation to the gradual lengthening was attained by sliding in Jess than 20% distraction but finally the muscle was regenerated by new muscle formation. 4. Major soft tissue complication to the gradual lengthening was induced by muscle. But arteries & nerves were well adapted to the gradual lengthening up to 30% Iengthening.