1. Captopril given for 5 days caused a fall in blood pressure in normotensive subjects. The percentage fall in mean supine pressure was greatest on a low sodium diet (10 mmol/day), 19.6%, least on a high sodium diet (350 mmol/day), 11%, and in between on a normal sodium diet (120 mmol/day), 16.5%.

2. Captopril caused a marked fall in plasma aldosterone in normal subjects on all three sodium intakes.

3. Captopril caused an increase in sodium excretion on the normal (120 mmol/day) and low (10 mmol/day) sodium diet but not the high sodium diet.

4. These results suggest that the renin-angiotensin-aldosterone system is a normal mechanism for maintaining blood pressure and aldosterone secretion in normotensive man. The system may also be involved in the maintenance of sodium balance.

5. These results may lead to a better understanding of the role of the renin-angiotensin-aldosterone system in the maintenance or causation of high blood pressure in essential hypertension.

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