The rate at which Ca45 administered as an oral dose appears in blood and bone of chicks is markedly affected by vitamin D. Absorption of Ca45 from an intramuscular dose is not influenced by either vitamin D or the calcium content of the diet. The interaction of the amount of Ca45 in the oral dose and vitamin D on the amount of calcium absorbed indicated that the vitamin D effect on the absorption is a function of the quantity of calcium presented for absorption. Absorption experiments carried out with chicks which were fed diets of varying calcium content showed that vitamin D enabled the chicks to adapt the capacity to absorb calcium to different dietary intakes of calcium.