I . A combination of balance and isotope techniques was used to determine the influence of phosphorus depletion on the size of the exchangeable calcium pool and on the rates of Ca deposition in and removal from the whole skeleton of sheep. 2. The exchangeable Ca pool was reduced in size in the depleted sheep to approximately 50 yo of that in the controls. 3. The rates of Ca deposition in and removal from bone were reduced by P depletion and the rate of Ca transfer from the pool was reduced slightly. 4. The turnover rates of the exchangeable Ca pool and bone Ca in sheep appear to be similar to values published for man. Recently Bronner, Richelle, Saville, Nicholas & Cobb (1963), using a kinetic approach to the study of Ca metabolism, showed that a significant positive correlation existed between the rates of Ca absorption from the intestine and Ca deposition in the bone. The relationship suggests that at high rates of intestinal absorption a relatively large amount of Ca was deposited in the skeleton. However, as Bronner (1964) points out, Ca absorption may not be an independent variable and may vary with the intensity of Ca metabolism. Support for this view might be obtained from the studies of Kimberg, Schachter & Schenker (1961), who showed that low-Ca diets enhanced Ca transfer in preparations of everted gut sacs from rats.The influence of a low P intake together with an adequate concentration of dietary Ca and vitamin D has not been studied by means of a kinetic approach, as used in the investigations cited above. Since ruminant diets usually supply adequate Ca but are often limiting in P (for review see Hill, 1961), it appeared desirable to obtain information on the influence of P depletion on the rates of Ca deposition and removal from the skeleton. Therefore, during a detailed investigation of the influence of P depletion on the absorption and excretion of Ca and P in sheep, the results of which Present address :
