AbstractNormal, rachitic, and vitamin D3‐replete chicken growth plates were studied utilizing the potassium pyroantimonate‐osmium tetroxide procedure. A marked membrane and mitochondrial calcium was revealed in all specimens in the maturing and early hypertrophic zones which disappeared as heavy matrix mineralization began.The most significant difference shown in the specimens was in the marked intracellular lipid content of chondrocytes in all zones of the rachitic and D3‐replete growth plates. There was negligible lipid present in normal specimens.It is suggested that as most of the mechanisms postulated as necessary for calcification are present in rachitic chicks, perhaps the increased intracellular lipid pool results from the formation of abnormal lipids for insertion into the plasma membrane and thus prevents normal calcium transport.Chains of intracellular vesicles were also visualized in maturing and hypertrophic chondrocytes. These were more often seen in rachitic growth plates and in increased numbers in the early D3‐replete specimens. The etiology is unknown at the prese