The purpose of this paper was to investigate long bone changes in silicon deficiency more extensively and under a new set of conditions. Long bone abnormalities have been produced in silicon-deficient chicks fed a casein-based rather than amino acid-based diet and under an entirely new set of conditions. As demonstrated previously feeding amino acid diets, the long bones of cockerels fed a silicon-supplemented basal diet and sacrificed at 4 weeks had a significantly greater amount of articular cartilage and water content as compared with the silicon-deficient group. Biochemical analyses of tibia for bone mineral, non-collagenous protein, hexosamine and collagen demonstrated that tibia from supplemented chicks had a significantly greater percentage and total amount of hexosamine and greater percentage of collagen than deficient chicks, the difference being greater for hexosamines than collagen. Tibia from silicon-deficient chicks also showed marked lesions, profound changes being demonstrated in epiphyseal cartilage, especially striking in the proliferative zone. The disturbed epiphyseal cartilage sequences resulted in defective endochondral bone growth indicating that silicon is involved in the metabolic chain of events required for the normal growth of bone.