Summary 1. Young calves are susceptible to rickets when solar ultraviolet radiation or some form of vitamin D is lacking in their dietary régime. 2. When the concentrations of calcium and inorganic phosphorus in the plasma are determined at weekly intervals, the progressive downward trend of either or both of these constituents usually preceded all other outward evidence. 3. The deficiency of radiant energy was also manifested by anorexia, by cessation of or decrease in the rate of growth, stiffness and bowing of the forelegs and finally, by the reduction of the ash and mineral content of the moisture-free, fat-free rib. 4. The exposure of rachitic calves to early spring sunshine caused the concentrations of calcium and inorganic phosphorus in the plasma to increase markedly. There was also a significant increase of these constituents in normal calves during the summer months and a corresponding decrease during the winter months. 5. These experiments demonstrate the effectiveness of solar ultraviolet radiation in enabling or permitting the calf to utilize more economically the materials present in the ration but not available to the body without the benefit of radiant energy or a supplementary form of vitamin D.