Plasma aldosterone, plasma renin activity, sodium and potassium in the plasma and the urine were determinated under acute stimulation with saline-depletion (furosemide) and under acute suppression with saline infusion in 40 patients with primary hypertension stage I, 19 patients with primary hypertension stages II and III, and 11 patients with renal hypertension (chronic glomerulonephritis and chronic pyelonephritis). The majority of the patients with primary hypertension stage I showed a good stimulation of the plasma aldosterone and the plasma renin activity under acute salt depletion. Three out of the 40 patients with primary hypertension stage I, and 13 of the 19 patients with primary hypertension stages II and III did not show any stimulation of the renin secretion ("low renin hypertension"). In all these patients the plasma aldosterone stimulation remained intact. With infusion of saline all the groups showed suppression of the plasma aldosterone and the plasma renin activity. A good stimulation of the plasma renin activity, demonstrates that in our experiments the renin-angiotensin system cannot be responsible for the increase in aldosterone secretion under salt depletion. Most likely the increase of the plasma aldosterone, in spite of the fixed renin activity, is stimulated by the sodium depletion due to diuretics. In all patients with primary hypertension we did not find an inadequate reaction of the aldosterone secretion under saline infusion. The patients with renal hypertension showed a minimal stimulation and suppression of the plasma renin activity. The plasma aldosterone secretion increased only slightly under sodium depletion and the decrease under saline infusion was statistically not significant. Thus we conclude that these patients show an inadequate reaction of the plasma aldosterone and renin secretion under salt infusion and depletion.