Comparison of the fall in forearm blood flow in response to intra-arterial norepinephrine (NE) in hypertensive and normal patients indicates that in hypertension the arterioles are twice as sensitive to NE as in health. The difference is further exaggerated by the initial constriction of the arterioles in hypertension. The hypertensives showed a similar hypersensitivity to 5-hydroxy-tryptamine and angiotensin. Similar experiments in children of hypertensive and normotensive parents suggest that the hypersensitivity is present in some children of hypertensive parents, at a time when the blood pressure is normal.