The effects of 5 days low sodium diet and diuretic were studied in 7 normotensive acromegalics, 8 hypertensive acromegalics and 12 normal subjects. Plasma renin activity was significantly lower in both groups acromegalics, compared with that of normal subjects. Plasma aldosterone was similar in normotensive acromegalics and healthy controls. The hypertensive acromegalics showed increased plasma aldosterone levels and blunted responses of plasma renin activity and plasma aldosterone to sodium depletion. Urinary prostaglandin E2 was significantly lower in hypertensive acromegalics before and after sodium depletion in comparison with normal subjects. Urinary 6-keto prostaglandin F1 alpha and thromboxane B2 were similar in all groups studied. The decreased production of PGE2 could contribute to the development of arterial hypertension in actomegaly.