1. The role of the plasma sodium concentration in the regulation of aldosterone secretion and sodium excretion was investigated by comparing in 13 conscious sodium-depleted dogs the effects of the same sodium load (2.5 mmol/kg) given as either a hypertonic or hypotonic infusion. 2. The plasma sodium concentration was significantly higher and the plasma aldosterone concentration and urinary aldosterone excretion were significantly lower after the hypertonic infusion as compared with the hypotonic infusion. 3. The cumulative urinary sodium excretion during the 22 h after beginning the infusion was significantly greater after the hypertonic infusion, but this difference was not observed in five sodium-depleted dogs who were treated with deoxycorticosterone acetate before the infusions were given. 4. These data suggest that elevations in plasma sodium concentration are effective in decreasing aldosterone secretion and, hence, in increasing sodium excretion in conscious sodium-depleted dogs.