Thirty-four patients with untreated Conn's syndrome were studied in a metabolic ward. The final diagnosis in each case was based on the finding and removal of an adrenal cortical adenoma with histological features typical of the disorder. Compared with 34 age and sex-matched normal controls the untreated patients had increased plasma aldosterone concentration, increased blood pressure (183/112 mmHg), increased exchangeable sodium (116.7% of normal), hypokalaemia and increased plasma sodium concentration. Exchangeable potassium was lower than normal and plasma concentrations of active renin, total renin and angiotensin II were lower than normal mean values. Arterial pressure correlated significantly and positively with plasma and exchangeable sodium and there was a significant negative correlation with plasma potassium concentration. Partial regression analysis showed that the relation of exchangeable sodium with blood pressure did not depend on age or renal function but that the relation of blood pressure and plasma potassium could be attributed to the correlation of exchangeable sodium and blood pressure. Multiple regression analysis suggested that exchangeable and plasma sodium were the most important determinants of blood pressure in untreated patients. Spironolactone, amiloride and surgical removal of the adenoma corrected the electrolyte abnormality and usually lowered blood pressure. The fall in exchangeable sodium was related to the fall in blood pressure. The pattern of correlation found by multiple regression analysis in postoperative patients was similar to that in normal subjects. The findings are relevant to some of the mechanisms proposed for the hypertension of mineralocorticoid excess.