Abstract Plasma renin activity was measured in 115 patients with hypertension. Increased plasma renin activity is consistently observed in patients with malignant hypertension and in patients in whom the serum sodium is depressed. Except in such patients, increased plasma renin activity is uncommon in hypertension and has little diagnostic or pathogenetic significance. In a series of Negro hypertensive patients 53 per cent were found to have no demonstrable plasma renin activity in casual blood samples. The significance of this finding is discussed, with special reference to possible primary aldosteronism.