SUMMARY: The effect of a low dose infusion of aldosterone (7·0–8·4 ng/kg/min) on urinary sodium and potassium excretion was assessed in six normal subjects. Plasma aldosterone levels during these infusions (mean 38, range 23–62 ng/100 ml) compared favourably with levels obtained during Na+ deprivation (mean 38, range 30–46 ng/100 ml). In four paired experiments, aldosterone infused in this dose for 2 h produced a significant reduction in Na+ excretion (P < 0·01) and urine volume (P < 0·05) in the 4th hour after the start of the infusion compared with the control infusions. There was no significant change in K+ excretion. Prolonging the aldosterone infusion reduced Na+ excretion. A 10 h infusion of aldosterone (7·7 ng/kg/min) in one subject reduced Na+excretion to 6 μequiv./min. These results are compared to the levels of Na+ excretion obtained in dietary Na+ deprivation.