Drill holes and fibular grafts were placed across the epiphyseal line of the distal end of the femur in thirty-eight rabbits and drill holes alone were made across the epiphyseal line in thirty-two rabbits. Microscopic sections at various times after surgery showed considerable new bone growing through the defects. When grafts were used they were fused by new bone to the epiphysis and metaphysis. Despite this, in the majority of cases a microscopic defect developed in the bone and epiphyseal growth continued. These bone defects or stress fractures are shown to be accompanied by an intense cellular reactions. The implications of these findings and their relevance to growth after fusion in young human beings are discussed.