PATIENTS who have a serum calcium level below 7 milligrams per 100 milliliters are known to have a greatly reduced urinary calcium excretion. The availability of 3 patients with low serum calcium values due to idiopathic hypoparathyroidism offered an opportunity to observe the effect of raising the serum level on the rate of urinary excretion of calcium. For this purpose, calcium gluconate was administered intravenously. We wish to present our observations on the changes in calcium excretion under these circumstances. A 3 per cent solution of calcium gluconate was prepared in the laboratory and 500 milliliters, containing approximately 1,350 milligrams of calcium, were injected intravenously at the rate of approximately 4 ml./min. Each study was performed in the morning, with the patients in the postabsorptive state. During the administration of the calcium, venous blood samples were drawn from the opposite arm at intervals of approximately one-half hour. Urine samples were obtained through an indwelling catheter at the intervals indicated in Tables 1–3. The bladder was washed out with 20 ml. of physiologic salt solution after each period, and complete emptying was assured by the injection of air into the bladder.