1. Mineral balance and radioisotope studies have been carried out to test the adequacy of recent Agricultural Research Council (1980) recommendations for calcium and phosphorus for pregnant and lactating ewes. At the same time, Ca metabolism was compared throughout pregnancy and lactation in ewes restricted to these recommendations and in ewes given a plentiful supply of dietary Ca and P.

2. Irrespective of their Ca intake, ewes were unable to absorb enough dietary Ca in late pregnancy and early lactation to meet the high demands, and skeletal stores of Ca were mobilized to make good the deficit.

3. Whereas ewes given the plentiful intake absorbed enough dietary Ca in mid- to late lactation to replace their lost skeletal Ca stores, ewes given the restricted Ca and P intake did not and, at the end of lactation, were still in deficit of about 10% of their total body Ca.

4. Since Ca absorption in the restricted ewes occurred at the maximum efficiency (66%) expected from a hay and concentrates diet, the Agricultural Research Council (1980) recommendations for Ca, particularly in mid-to late lactation, must be inadequate.

5. It is suggested that in calculating requirements, allowance must be made for the inevitable loss and subsequent replacement of skeletal stores.

6. The endogenous faecal loss of Ca was found to vary in direct relation to the food intake, and the significance of this finding to calculations of Ca requirements is discussed.