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)