Structural changes in bone, parathyroid, and ultimobranchial body were examined in three groups of chicks fed a vitamin D-deficient diet; one group was treated with vitamin D3 and another with 1,25(OH)2D3. Diets were fed from day of hatching until 5 weeks old, when deficient chicks were near death due to hypocalcemic tetany, loss of fat and muscle, and marked bone deformities. In deficient chicks, parathyroid mass increased linearly to 7.5 times normal at 5 weeks. Parathyroid cells were irregular and vacuolated, with few granules. 1,25(OH)2D3 had normal parathyroids until the fifth week, when parathyroid mass increased greatly. There were few differences in length of growth cartilage, but marked changes in length of metaphyses. Deficient chicks had metaphyses nearly five times longer than vitamin D3-treated chicks. Metaphyses in chicks given 1,25(OH)2D3 were twice as long as those of vitamin D-treated chicks at 5 weeks. Both osteoblasts and osteoclasts were more numerous in deficient chicks. These studies suggest that vitamin D3 is more effective than 1,25(OH)2D3 in preventing parathyroid and bone lesions of vitamin D deficiency.