We examined the antihypertensive response to enalapril and its relation to the changes of the renin-angiotensin-aldosterone axis (RAA) in essential hypertensive patients. The responders, whose blood pressure reduced after enalapril administration, had significant elevation of plasma renin activity after enalapril (7.5 +/- 4.0 to 31.9 +/- 10 ng/mL/h, p < 0.05). There was no significant change in the plasma renin activity of the nonresponders (3.0 +/- 1.0 to 5.6 +/- 1.9 ng/mL/h). Natriuretic response to acute salt loading was compared between the two subgroups. The natriuresis on acute salt loading of nonresponders was better than that of the responders (82.9 +/- 13.2 vs 44.5 +/- 5.7 mmol/4h, p < 0.05). These results imply that the RAA and extracellular fluid volume both contribute in different ways to the maintenance of high blood pressure in different subgroups of essential hypertension.