The response of the renin-angiotensin system to high and low sodium diets, to standing, and to saralasin infusion was assessed before and after surgical correction of aortic coarctation in a 27-year-old man. The cardiovascular responses to tests of autonomic function were measured. The heart rate responses to the Valsalva manoeuvre and standing were abnormal before operation, and plasma renin levels were high and renin secretion responded poorly to changes in dietary sodium, to standing, and to saralasin. Renin responsiveness and cardiovascular reflexes returned to normal after operation. The results are consistent with the hypothesis that there is a high level of sympathetic efferent activity in coarctation of the aorta and that factors other than increased activity of the renin-angiotensin system may cause high blood pressure.