1. The renal excretion of phosphate was studied in dogs under control conditions and during infusion of ethacrynic acid or frusemide at different levels of filtered load.

2. Both of these diuretic compounds significantly enhanced the fractional excretion of phosphate at normal and moderately decreased filtered loads, and a good correlation between fractional excretions of sodium and phosphate could then be demonstrated.

3. When the blood flow to the left kidney was decreased by constricting the aorta below the right and above the left renal arteries, fractional excretion of phosphate decreased below control levels in the left kidney although that of sodium remained above pre-diuresis values.

4. The phosphaturic effect of ethacrynic acid and frusemide might be the result of a proximal site of action. The reversal of the phosphaturia by aortic constriction might have been caused by enhanced proximal reabsorption of phosphate and minimal delivery of this ion to the more distal sites of diuretic action.

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