1. The effects of the infusion of a low dose (2 pmol min−1 kg−1 for 3 h) of human atrial natriuretic peptide (hANP) were studied in seven healthy volunteers undergoing a water diuresis. Lithium clearance was used to monitor proximal tubular function.

2. hANP increased urine flow rate, sodium, calcium and magnesium excretion without significant changes in potassium and phosphate excretion, heart rate or blood pressure.

3. hANP caused a small change in fractional lithium clearance, and larger changes in distal nephron handling of sodium and water.

4. Plasma renin activity tended to decrease during the infusion of hANP, while plasma aldosterone concentration decreased during and increased after stopping the infusion of hANP.

5. The data suggest that hANP inhibits the reabsorption of sodium and water by an action on distal segments of the nephron and perhaps the proximal tubule. Inhibition of renin and aldosterone secretion may contribute to the natriuresis.

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