Studies were performed with ticrynafen (tienilic acid) to determine its natriuretic site of action, detailed electrolyte excretion, and its effect on acid excretion and bicarbonate reabsorption. With 500 and 1,000 mg oral doses under conditions of water diuresis, there was a decrease in free water excretion as osmolar clearance increased. During hydropenia, there was no change in free-water reabsorption as osmolar clearance increased. Maximum sodium excretion reached 5.2% of the filtered load. At 500 mg doses, there was no effect on phosphate excretion; with 1,000 mg doses, there was a reduction in phosphate excretion. During bicarbonate infusion, there was no change in the absolute or percent reabsorption of bicarbonate. After chronic administration of ticrynafen, there was no impairment of excretion of al acute acid load. Uric acid excretion increased at least threefold in all studies. The studies indicate that at 500 and 1,000 mg oral doses, ticrynafen has the characteristics of a diuretic which is active in the cortical diluting segment of the distal nephron.