1. The effect of intravenous loading with 500 ml of sodium chloride solution (50 g/l) on plasma renin concentration, plasma aldosterone concentration, urinary sodium excretion and mean blood pressure was studied in 15 young patients with mild essential hypertension and 10 healthy normotensive control subjects. 2. Plasma renin concentration and plasma aldosterone concentration were suppressed to the same degree during loading in both the hypertensive and normotensive groups. Urinary sodium excretion was significantly higher in the hypertensive patients than in the normotensive subjects. Mean blood pressure increased slightly in both groups. 3. Plasma renin concentration and plasma aldosterone concentration were significantly correlated in both groups before sodium loading. The increase in urinary sodium excretion was significantly correlated to the suppression of plasma aldosterone concentration in the hypertensive, but not in the normotensive, group. No correlation was found between changes in urinary sodium excretion and changes in plasma renin concentration or mean blood pressure. 4. The results indicate that the suppressibility of the renin-aldosterone system by hyperosmotic sodium chloride solution is normal in young patients with mild essential hypertension. It is suggested that the changes in plasma aldosterone concentration induced by sodium loading might be involved in the regulation of exaggerated natriuresis in essential hypertension.