1. The effect of vasopressin on the tubular reabsorption of phosphate was studied in both water-restricted and water-loaded man at different baseline rates of phosphate excretion. The latter was varied by altering phosphate intake or by administering aluminium hydroxide to reduce phosphate absorption from the gut.

2. It was found that when phosphate excretion was high, vasopressin had no consistent effect on the phosphate/creatinine clearance ratio. However at low rates of phosphate excretion, vasopressin significantly increased the phosphate/creatinine clearance ratio, suggesting a decrease in net tubular reabsorption of phosphate.

3. This effect of vasopressin was also demonstrated in a patient with hypoparathyroidism, indicating that it is not mediated by release of parathormone.

4. It thus appears that vasopressin has a small parathormone-like effect on the renal tubules, which may result from the common action of both hormones on the adenyl cyclase system.

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