1. The effect of clonidine on the relationship between sympathetic nervous system activity and the state of sodium-volume balance was studied in 15 patients with essential hypertension and normal renal function (group 1) and in 14 patients with hypertension and mild to moderate renal failure (group 2).
2. Acutely, clonidine (200 μg) produced significant falls (P < 0.01) in mean blood pressure, plasma noradrenaline, plasma renin activity and plasma aldosterone in both groups of patients. The changes in mean blood pressure were significantly correlated (P < 0.01) with the changes in plasma noradrenaline.
3. Chronic therapy with clonidine also produced significant falls in mean blood pressure and plasma noradrenaline, but not in plasma renin activity or aldosterone.
4. Exchangeable sodium and plasma volume decreased significantly in patients of group 1 but not in patients of group 2.
5. The data indicate that sympathetic nerve activity may be important for the abnormal relationship between pressure and natriuresis in subjects with essential hypertension and normal renal function, but not in hypertensive subjects with impaired renal function.