1. The renin-aldosterone system was studied in seventy-one selected hypertensive patients. Nine (13%) were diagnosed as having primary aldosteronism. Of the twenty-three patients who presented with a history of unprovoked hypokalaemia, the incidence of primary aldosteronism was 40%.
2. Renin and aldosterone responses to the combined stimuli of a low sodium diet and the upright posture were suppressed in patients with essential hypertension. There was no evidence that the suppression was due to abnormal adrenal function, sympathetic neuropathy, or the level of the blood pressure. The mechanism of the suppressed plasma renin activity response and its significance in the pathogenesis of hypertension are unknown.