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 (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.