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Collip, Pugsley, Selye and Thomson1have observed resorption of bone in bilaterally nephrectomized rats which had been injected with massive doses of parathyroid extract. They are of the opinion that the primary action of parathyroid hormone is to cause an increased proliferation of osteoclasts, which actively function to release calcium from the bones to the blood. Since the observations of these workers were not supplemented by serum calcium determinations, which should yield evidence of the passage of calcium into the blood, we have carried out similar experiments†in which calcium analyses have been made.Immature and mature rats of both sexes have been used. The animals (Table I) were not given food, but were supplied with water ad libitumduring the 48-hour experimental period.† Parathyroid extract (Lilly)§ was injected immediately after nephrectomy, and successive doses were administered after 6, 12, and in some instances, 18 hours. Blood was obtained by cardiac puncture, without using an anesthetic, just before the animals were killed. Calcium analyses2were made by one of us (W.R.T.) on individual samples, which in most instances consisted of 2 cc. amounts of serum, and in no case less than 1 cc.The data in Table I show that the serum calcium level in the normal rat (Group A) is well maintained during the first 2 days of starvation. After nephrectomy (Group C) the serum calcium level has a tendency to drop, and during the same period we have found in other work3that the serum inorganic phosphate increased rapidly. When the calcium values obtained in Groups B, C, and D are compared it is observed that in Group D there is only slight indication of mobilization of calcium as a result of parathyroid hormone administration.