1. In 29 patients with acromegaly, plasma renin activity and growth hormone were measured during fasting and recumbency on free diet. Exchangeable sodium was measured in all cases and expressed as a percentage of the expected value on the basis of lean body mass.
2. Twenty-two control subjects without evidence of cardiovascular, renal or endocrine disease were studied in the same way.
3. There was a significant increase in exchangeable sodium and suppression of plasma renin activity in the acromegalic patients in comparison with control subjects.
4. There was a significant positive correlation between exchangeable sodium and plasma growth hormone.
5. Hypertensive acromegalic patients (diastolic blood pressure ≥ 100 mmHg) tend to have a lower (although not significantly so) exchangeable sodium than normotensive subjects.
6. We conclude that (a) suppression of plasma renin activity in acromegaly can be explained by sodium retention, (b) hypersecretion of growth hormone is probably responsible for the increased exchangeable sodium, and (c) sodium overload cannot be directly related to blood pressure but may contribute to the increased occurrence of hypertension in acromegaly.