1. The direct effect of calcium on the hyperparathyroidism of chronic renal failure was studied in rats with induced chronic renal failure, who were fed on a diet low in phosphate and who received supplemental phosphate by injection. They were given a normal (0·8%), or low (0·1%) or high (1·7%) calcium diet.
2. The animals on the low calcium diet had larger parathyroids and more severe bone disease at the end of 4 weeks, indicating the importance of calcium intake in directly influencing the degree of hyperparathyroidism.
3. Increasing the calcium content of the diet from 0·8% to 1·7% produced no additional benefits.