In the limb lengthening by gradual distraction techniques, permanent tissue damage and joint contracture or subluxation can occur if the bone is lengthened beyond a certain safety limit. The main cause of those problems seems to be the difference in the reaction to distraction between bone and muscle. The purpose of this study was to investigate the difference between the tibialis anterior, a monarticular muscle and extensor of ankle joint, and gastrocnemius, a biarticular muscle and flexor of knee and ankle joint, in their response to bone lengthening in relation to the percentage of lengthening of the tibia by callotasis. Fifteen growing rabbits were divided into three lengthening groups; group I (10% lengthening), group II (20% lengthening), and group III (30% lengthening). The lengthening was done on the left tibiae and the right tibiae were used as a control. Length of the muscle belly, length of the tendon, and weight of the muscle were measured. Histopathologic studies by hematoxylin eosin and Masson trichrome stain were done on the mid-portion of muscle belly and distal musculotendinous junction of each muscle. Scoring system based on five parameters (atrophy of muscle fibers, the internalization of nuclei of muscle fibers, the degeneration of muscle fibers, the regeneration of muscle fibers, and the endomysial fibrosis of muscle fibers) was utilized for semi-quantitative analysis of histopathologic study. The following results were obtained.; 1. The length of muscle belly significantly increased in the experimental side compared to the control in group II, group III of tibialis anterior and in group III of gastrocnemius (P<0.05). 2. The length of tendon did not significantly increase in the experimental side compared to the control in any group of tibialis anterior and gastrocnemius (P>0.05). 3. The weight of muscle did not significantly increase in the experimental side compared to the control in any group of tibialis anterior and gastrocnemius (P>0.05). 4. The score of any 5 parameters did not significantly increase in the experimental side compared to the control in the mid-portion of muscle belly and musculotendinous junction of tibialis anterior and gastrocnemius (P>0.05). 5. The sum of 5 parameters did not significantly increase in the experimental side compared to the control in the mid-portion of muscle belly and musculotendinous junction of group I of either tibialis anterior or gastrocnemius (P>0.05), but significantly increased in the mid-portion of muscle belly and musculotendinous junction in group I and group II of both tibialis anterior and gastrocnemius (P<0.05). The results suggest that significant histopathologic changes can occur beyond 20% lengthening in tibialis anterior and gastrocnemius, and that gastrocnemius is less compliant to distraction than tibialis anterior.