1. Plasma immunoreactive atrial natriuretic peptide (ANP) and urinary sodium excretion were measured in six normal male subjects before, during and for 195 min after a 60 min infusion of 2 litres of saline (0.9% NaCl, 308 mmol of Na+).
2. During the saline infusion, there was a significant increase in plasma ANP and urinary sodium excretion and a significant decrease in plasma renin activity, aldosterone, albumin, creatinine and packed cell volume.
3. The maximal rise in mean plasma ANP occurred 15 min after stopping the infusion and the maximal rise in mean urinary sodium excretion in the collection period 30 min later.
4. Plasma ANP then decreased so that by the end of the study the level was the same as before the saline infusion. However, at this time, 195 min after the saline infusion was stopped, there was still a net positive sodium balance of 220 mmol and urinary sodium excretion remained significantly elevated.
5. Our results are compatible with the concept that increased ANP secretion may play a role in the immediate increase in sodium excretion after a saline load. However, they also suggest that other mechanisms may be more important for the longer term increase in sodium excretion.