1. Suppression of the renin—aldosterone system by expansion of the extracellular fluid volume with extra sodium and mineralocorticoid for 6 days was studied in nine young men with very mild essential hypertension and in ten normotensive young men.
2. Plasma renin activity, measured both supine and after 45° head-up tilt, and the renal excretion of aldosterone 18-glucuronide were similar in both groups. However, after expansion of the extracellular fluid volume, hypertensive patients showed much less suppression of both variables.
3. This difference persisted despite matching for an equivalent degree of expansion of the extracellular fluid volume as indexed by the change in body weight.
4. Administration of extra sodium and mineralocorticoid produced a greater proportional fall of renal aldosterone excretion than of plasma renin activity in both groups and this dissociation was significantly more marked in the hypertensive group.
5. We suggest that (i) a relative autonomy of the renin—aldosterone system may be relevant to the pathogenesis and/or perpetuation of essential hypertension and (ii) that the syndrome of low-renin hypertension is unlikely to be associated with ‘mineralocorticoid’ excess.