Three series of long period feeding experiments with cattle are recorded, during which balances of Ca and P for the individual animals were determined. The first experiment was designed to test the availability of various types of phosphate supplement, with varying amounts of CaCO3, when fed at a total P intake level well below the requirement of the particular class of animal used and with a constant Ca: P ratio of 2: 1. Disodium phosphate was slightly more available than either dicalcium phosphate or bonemeal, the relative retentions being 100, 97 and 94 per cent. of the P supplement. The retentions of Ca varied from 65 to 100 per cent. In a second experiment rations containing 8. 5 g. P and 16. 4 g. Ca daily were found to be adequate for steers whereas intakes of 5. 8 g. and 12. 0 g., respectively, were insufficient when tested by blood and bone analyses and Ca and P balances. It was considered that the percentage of ash in the fresh bones was an excellent index of calcification and paralleled the breaking strength of the metacarpus for animals of the same age. In the third experiment, both the levels of P and Ca and the Ca: P ratios were varied over wide ranges for the various groups of heifers. A daily intake of 23 g. Ca and 10. 5 g. P, giving average daily retentions of 9. 7 g. Ca and 6. 6 g. P, was found to be adequate for growing heifers on the bases of blood and bone analyses and Ca and P balances. At this level of P, lowering the Ca intake to 4. 1 g. reduced the P balance more than did raising the Ca intake to 45. 0 g. It is suggested that the main factor which determines retention of Ca or P is the level of intake of the element in question, apart from the ratio in which it is associated with the other mineral element in the food.