1. Two normal subjects and ten patients with chronic renal failure were given 15 or 20 g of calcium carbonate in the morning and 5·6 or 8·4 g of calcium phosphate in the evening for 13–41 days.
2. During the high calcium and phosphate intake there was a rise in calcium and phosphate absorption from the gut in the normal subjects and to about the same extent in the patients with chronic renal failure.
3. The calcium and phosphate balances became positive while there was a rise in plasma calcium and a fall in plasma phosphate. There was also a fall in urinary phosphate excretion.