Reports on the effects of physeal distraction on chondrocyte proliferation are controversial, and the biologic mechanism of lengthening is still open. Proliferation of the growth plate chondrocytes was studied after gradual physeal distraction in 12 growing rabbits. Distraction of the distal femoral physis was performed with an external fixation device for three to 14 days. Proliferating cells were labeled with 5-bromo-2'-deoxyuridine one hour before killing the animal, and labeled chondrocytes were localized with a specific monoclonal antibody. Labeled chondrocytes were observed in the hypertrophic zone after distraction of three to seven days, in addition to labeling in the proliferative zone; in the remaining physes, the distribution of labeled cells was normal. Physeal widening occurred in all distracted specimens, and a fracture-separation, usually through the hypertrophic zone, in all but one. The labeled hypertrophic cells were seen on the epiphyseal side of the separation gap, five to 20 cells in each specimen. Although hypertrophic chondrocytes are known to be metabolically active, proliferation of these cells has never been reported before. Hypertrophic chondrocytes seem capable of proliferation under special circumstances.