Longitudinal bifurcation osteotomy extending through the distal epiphysis, growth plate, and metaphysis of the radius and ulna of young rabbits and dogs was performed and an inert plastic membrane was placed in the osteotomy. The degree of resultant deformity appeared to be in direct proportion to the mass of the osseous epiphyseal centrum present at the time of surgery. When the mass of the osscous epiphyseal centrum was large, early epiphyseodesis and deformity occurred as a consequence of new-bone formation bridging between the epiphysis and the metaphysis. When the osseous epiphyseal centrum was small at the time of surgery and a relatively large portion of epiphyseal cartilage separated the osteogenic sites (the epiphyseal centrum and the metaphysis), relatively normal growth continued until the centrum came to rest against the growth plate. Epiphyseodesis and deformity then occurred. When the bifurcation was performed prior to the appearance of the osseous centrum, relatively normal undeformed growth of the divided halves occurred.