A survey is given on the present state of the animal-biochemical research concerning the effectivity of the D-vitamins. The vitamin D3 is transferred in 25-hydroxycholecalciferol in the microsome fraction of the liver. Then in the kidneys a transformation into the physiologically active forms - the 1,25- and the 24,25-hydroxycholecalciferol - takes place. The kind of transformation depends on the Ca- and P-supply as well as on the level of the parathormone secretion. In the mucous membrane of the small intestine the 1,25- and the 24,25-hydroxycholecalciferol stimulate the formation of the Ca-transport protein. The 1,25-hydroxycholecalciferol is above all formed in hypocalcaemia or hypophosphataemia and furthers the mobilisation of Ca- and phosphate-ions from the bones. In a liver damage disturbances of the formation of 25-hydroxycholecalciferol and in renal damage disturbances of the formation of 1,25- and 24,25-hydroxycholecalciferol may appear. A reduction of the formation of the active forms of the D-vitamins leads to a decrease of the Ca-utilisation as well as of the growth of bones.