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The kinetics of plasma calcium regulation in man were studied by noting the response to intravenous injection of calcium as gluconate given at the rate of 10 mg/kg/hr. Twenty-nine infusion tests were carried out on 20 adult male subjects. The series comprised normals, cases of hypoparathyroidism, hyperparathyroidism, Paget's Disease, and osteoporosis. All were hospitalized at the time of the study. The changes in levels of plasma calcium, phosphorus and citrate were followed by frequent blood sampling. Excretion of calcium, phosphorus and citrate by the kidney was also determined. In all cases, plasma calcium levels rose in response to the infusion and declined slowly after it had ended, returning to normal several hours later. Plasma phosphate and citrate also rose slightly, but more slowly than the plasma calcium, and remained elevated as long as plasma calcium was above normal. The increase in the calcium excretion rate was directly proportional to the plasma calcium during the period of hypercalcemia. From this linear relationship it was possible to estimate a tubular maximum (Tm) for reabsorption of calcium by the renal tubules. The value obtained for calcium Tm in this series of 8 normal subjects was 4.97 ± 0.27 mg/min (range 3.60 – 5.80). In the one case of osteoporosis studied, the Tm appeared to be increased following correction of a hyperchloremic acidosis. The increased calcium excretion during hypercalcemia is a most important homeostatic factor in restoring the plasma level to normal. The rapidity of the exchange between the bone and the soft tissue calcium compartments appeared to be much greater in the cases of Paget's disease, as might be predicted from the high rate of bone blood flow characteristic of this disease.