An experimental study on physeal distraction was carried out in the distal femur of 45 two-month-old lambs for the purpose of identifying the basic mechanism of lengthening as well as assessing growth cartilage viability after lengthening. The animals were divided into three groups, each with three subgroups, according to the distraction rate employed (2 mm/day, 1 mm/day, 0.5 mm/day) and the time at which the animals were killed (end of lengthening, 1.5 months postlengthening, and at six months of age). Another group of three animals whose femurs were lengthened at a rate of 0.5 mm/day was killed ten days postoperatively. Roentgenologic, specimen measurements, and histologic studies were performed on all animals. The results obtained showed that the basic mechanism for lengthening is the production of a physeal fracture between degenerative and calcified layers; this finding was consistent. It was also observed that the lower the rate of distraction employed, the greater was the short- and long-term viability of the growth cartilage. More specifically, optimal viability was observed when a distraction rate of 0.5 mm/day was employed.