1. Arterial pressure and exchangeable sodium (NaE) were measured in patients with Conn's syndrome, essential hypertension, renal artery stenosis and chronic renal failure. Comparison was made with a control group. Urine sodium excretion was measured separately from the two kidneys in patients with renal artery stenosis.

2. Compared with control, mean NaE was significantly increased in Conn's syndrome, and was normal in essential hypertension, renal artery stenosis and chronic renal failure.

3. The correlation of arterial pressure with NaE was positive and significant in Conn's syndrome, essential hypertension and chronic renal failure.

4. In contrast the correlation was significantly negative in unilateral renal artery stenosis. Patients with lowest NaE had hyponatraemia, hypokalaemia and secondary hyperaldosteronism.

5. Urinary sodium excretion from the unaffected kidney in unilateral renal artery stenosis correlated positively with arterial pressure, possibly reflecting the phenomenon of pressure-natriuresis. Patients subsequently responding least well to surgery excreted least sodium from the untouched kidney for a given arterial pressure.

6. The findings suggest important roles for arterial pressure in the regulation of sodium balance (predominant in renal artery stenosis), and for sodium balance in the regulation of arterial pressure (predominant in Conn's syndrome). The observations in essential hypertension are compatible either with an exact balance between these mechanisms or with the existence of some other mechanism raising blood pressure.

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