The epiphyseal plates of normal rats, of rats made rachitic by a diet deficient in vitamin D, and of rachitic rats treated for six days with vitamin D₂ were studied autoradiographically after administration of thymidine-³H, cytidine-³H, glycine-³H, and Na₂³⁵SO⁴. The data obtained demonstrated that:

1. Thymidine-³H incorporation (indicating DNA synthesis) occurred chiefly in the proliferative zone and was only minimally affected by the rachitic state or during recovery with Vitamin-D therapy. In animals with florid rickets, a secondary zone of DNA synthesis was noted in the lowermost portion of the maturation zone.

2. Incorporation of cytidine-³H (indicating RNA synthesis), glycine-³H (indicating protein synthesis), and ³⁵SO₄^— (indicating polysaccharide synthesis) was qualitatively unchanged in the proliferative zone of the rachitic animals but was profoundly altered in the maturation zone. A marked decrease in autoradiographic localization of the isotopes in this area indicated a virtual suspension of synthetic activities. With vitamin-D therapy, the isotopic labels were seen to increase rapidly on the epiphyseal side, the increase sweeping toward the metaphysis as the duration of treatment increased. This manifestation of healing is in direct contradistinction to the calcification and bone formation which begin in the metaphysis and move toward the epiphysis.

3. The relationship of these observations to alterations in intermediary metabolism and energy production were discussed.