1. Systemic, humoral and renal responses to isotonic volume expansion (1800 ml in 3 h) were assessed in normal subjects and patients with normal renin essential hypertension before and during captopril administration.
2. Essential hypertensive subjects had a greater natriuretic and diuretic response to volume expansion than had normotensive subjects.
3. Captopril induced a fall in pre-saline mean arterial pressure more marked in hypertensive (20 ± 3 mmHg) than in normotensive subjects (9 ± 2 mmHg) and did not produce any change in sodium balance.
4. Captopril exaggerated the response of arterial pressure to volume expansion since mean arterial pressure increased more markedly after than before captopril in both normotensive (18.7 ± 3.8%) and hypertensive subjects (16.9 ± 3.7%).
5. Captopril blunted the exaggerated natriuretic response to volume expansion observed in patients with essential hypertension, whereas the renal response was unchanged in normotensive subjects.