Acute response in blood pressure (BP) and natriuresis to saline infusion was evaluated in 16 patients with primary aldosteronism caused by aldosteronoma (PA) and 12 patients with salt-sensitive essential hypertension (SSEH). Salt-sensitivity was defined by a decrease in mean BP exceeding 5% at the second hour after a 20 mg furosemide injection. Plasma renin activity (PRA), plasma aldosterone concentration (PAC) and urine electrolytes in response to saline infusion were determined. During a 2-liter isotonic saline infusion, a similar degree of natriuresis and change in BP were observed in PA and SSEH patients. A significantly inverse correlation between the increase in mean BP and the degree of natriuresis at the end of the infusion was found in patients with SSEH (r = -0.80, p less than 0.01). No correlation was observed between these parameters in patients with PA (r = 0.28, p greater than 0.05). These results suggest that hypernatriuresis in SSEH may play a protective mechanism against abrupt increases in BP and volume during acute saline loading. This protective mechanism was not evident in patients with PA.