The physis has limited ability to undergo repair, and injury may result in growth arrest. Osteogenic protein-1 promotes bone formation in diaphyseal defects, chondrocyte proliferation, and matrix synthesis. The authors' goal was to determine if the presence of osteogenic protein-1 in a defect involving the physis would promote cartilage repair, and in doing so, to determine the effect of osteogenic protein-1 on physeal growth. An ovine model of growth plate damage was used, in which the proximal medial physis of the tibia was partially ablated. The defect was filled with a Type I collagen paste containing osteogenic protein-1 (350 microg) or collagen alone. Growth rate was measured at 4, 14, and 56 days, and the defects were analyzed histologically at 4, 14, and 56 days. Bone bridge formation occurred within the defect site. However, osteogenic protein-1 promoted outgrowth of the adjacent physeal cartilage. The physeal cartilage underwent expansion until the mineral forming within the defect site blocked its progress. The effect was localized because only that portion of the physis at the defect margin appeared to be affected.