Exchangeable and plasma electrolytes, blood pressure and aldosterone were measured in groups of patients with idiopathic hyperaldosteronism, primary hyperaldosteronism and essential hypertension and in normal subjects. In idiopathic hyperaldosteronism exchangeable sodium was higher than in both essential hypertensive and normal groups but lower than in primary hyperaldosteronism. Plasma sodium results were similar except that no difference existed between the two forms of hyperaldosteronism. Plasma potassium concentration was lower in idiopathic hyperaldosteronism than in either essential hypertensive or in normal groups, but higher than in primary hyperaldosteronism. Blood pressure correlated with age in all groups and with exchangeable sodium in hypertensive patients. This was also the case with exchangeable sodium:exchangeable potassium ratio, but blood pressure did not correlate with aldosterone in any group. In idiopathic hyperaldosteronism, as in essential hypertension, sodium and blood pressure correlated strongly in male and weakly in female patients. The analysis reveals important differences between idiopathic and primary hyperaldosteronism and also between idiopathic hyperaldosteronism and essential hypertension.