Limb lengthening is now an accepted form of correcting limb length inequality. It is, however, associated not infrequently with a variety of complications including pheripheral nerve palsies. Such nerve dysfunction is obviously dependent on the velocity and amount of bone lengtening. In this experiment, the author attempted to determine the safe limits of the velocity and amount of bone lengthening by measuring somatosensory evoked potential (SSEP). The tibiae of 96 rabbits, weighing from 1.5 to 1.6kg, were osteotomized; in Group I, the tibiae were lengthened 0.35mm per day once daily;in Group II, 0.7mm per day; in Group III, 1.05mm per day; and in Group IV, 1.4mm per day. Several rabbits were subjected to the study weekly until, six weeks postoperatively in Group I, five weeks postoperatively in Grup II, four weeks postoperatively in Group III, and three weeks postoperatively in Group IV. The following results were obtained: 1. Preliminarily, in order to verify the possible difference between the right and left tibiae, Pl latency and amplitude were measured in 18 rabbits. Paired t-test revealed no statistically significant difference between the two (p>0.05). The amplitude manifested more individual variation than the Pl latency. 2. As lengthening progressed, the Pl latency gradually increased, whereas the amplitude decreased. The higher the percentage of lengthening to the original tibial length, the more marked the changes in both the Pl latency and amplitude. 3. As lengthening progressed, significant changes were observed earlier in the amplitude than in the Pl laterncy. 4. Significant changes in the amplitude were observed when lengthening reached 12.2% of the original tibial length in Group I (0.35mm/day) and 10.5% in Group IV (1.4mm/day). Significant changes in the Pl laterncy were observed when lengthening reached 17.8% of the original length. It is concluded that somatosensory evoked potential is an effective method of detecting early and preventing neurological complications in bone lengthening. Somatosensory evoked potential is a clinically feasible technique. It is expected that the finding of study may serve as a useful guideline for detecting the safe limits of velocity and amount of bone lengthening.