Factors affecting the level of urinary excretion of phosphorus following a four-hour intravenous infusion of calcium were examined. Changes in amount of urinary phosphorus excretion during the second half of the calcium infusion day, were compared to those of the corresponding period of the control day in 20 patients with hyperparathyroidism and 22 normal subjects. Responses to calcium infusion differed in the two groups of subjects. In normal patients phosphorus excretion was decreased by > 29%, in hyperparathyroid patients by < 25%. Since mechanisms other than parathyroid depression may decrease phosphorus excretion during the first 12 hours after calcium infusion, the second 12-hour period is more suitable for evaluating parathyroid function.