The important role of the epiphyseal cartilage in the longitudial growth of long bones is universally accepted. Injury to the epiphyseal plate caused by surgically produced defects or trauma has been observed clinically and in experimental investigations. These growth retarding effect of defects seem to be proportional to the size of the defect, and are caused by the formation of a bone bridge connecting the epiphysis to the metaphysis. This investigation provides an explanation for the intimate relationship between the size of the defect and growth retarding effect. Central destruction of the distal femoral epiphyseal plates on 81 immature rabbits were performed by four different sized drills. Free flaps of vastus lateralis were inserted into the defect of the epiphyseal plate. Left limbs were used as the experimental group, and right limbs as the control. After 8 weeks of operation, animals were sacrificed, and roentgenological and histological observations were made. The results were observed as follows. 1. The amount of bony bridge connecting epiphysis and metaphysis was proportional to the size of the epiphyseal defect. 2. The amount of growth retardation at the distal femoral epiphyseal plate was proportional to the size of the defect produced in the epiphyseal plate. 3. Compensatory overgrowth at the proximal femoral epiphysis seems to be proportional to the size of the defect made in the epiphyseal plate. 4. Insertion of the free flap of vastus laterials into the epiphyseal defect diminished the formation of the bony bridge.