Summary1. Studies of plasma renin concentration in human hypertension have demonstrated an inverse relationship between renin and plasma sodium concentration. For reasons which have been discussed this work has not excluded the possibility that renin is present in the blood of hypertensive patients in amounts which are sufficient to affect blood pressure.2. Plasma renin determination may be of value in the differential diagnosis of patients with primary hyperaldosteronism and the hyponatraemic hypertensive syndrome. It may also be valuable in assessing the prognosis of renal artery surgery. In both situations it remains to be shown whether or not renin estimation is potentially more informative than repeated and accurate measurement of plasma sodium.