When a physiological dose of cholecalciferol was given to rachitic chicks, an early response was increased amino acid incorporation into growth cartilage protein, including collagen, but no evidence was found for induction of specific protein synthesis as in the intestine. Increased release into the medium of 45Ca and of hydroxyproline was observed when growth cartilage tissue was incubated in medium containing 1,25-dihydroxycholecalciferol. These studies suggested that the metabolism of cartilage tissue from the zones of proliferation and maturation was more affected than tissue from the zones of hypertrophy and calcification. Cholecalciferol metabolites may therefore have a direct effect upon chondrocytes as well as on bone cells during endochondral bone formation