The biological activities of vitamin D₃, 25–hydroxyvitamin D₃, and 1,2 5–dihydroxyvitamin D₃ given each day ip to rats have been compared. Under these conditions, 1,25–dihydroxyvitamin D₃ is about 10 times more active than vitamin D₃ in the elevation of serum phosphorus and calcification of bone of rats on a low phosphorus diet. It is also about 5 times more active than vitamin D₃ in the maintenance of serum calcium and stimulation of growth of rats on a low calcium diet. The 25–hydroxyvitamin D₃ equals 1,25–dihydroxyvitamin D₃ in its effect on serum calcium and growth of rats on the low calcium diet, but is 1/5 as active in antirachitic activity in rats on the low phosphorus diet. In addition, 1,25–dihydroxyvitamin D₃, but not vitamin D₃ or 25–hydroxyvitamin D₃, tends to produce a hypercalcemia even at low doses in rats on a high calcium diet. (Endocrinology92: 417, 1973)