A recent review of 25 patients who had repair of coarctation of the aorta at the Milton S. Hershey Medical Center revealed a 56% incidence of paradoxical hypertension in the immediate postoperative period. Analysis of results from recent animal experiments helped us to formulate a logical explanation of the pathogenesis of this hypertension. Stimulation of sympathetic nerve fibers located between the media and adventitia of the aortic isthmus has two effects, both of which result in hypertension. The first effect is release of norepinephrine and consequential rise in systemic blood pressure. Second, this spinal reflex directly stimulates the juxtaglomerular cells to release renin and cause additional hypertension. An additional effect of this increased renin production might be the shunting of blood from mesenteric arteries, thus causing the abdominal symptoms of so-called post-coarctectomy syndrome. The inability of some patients to adapt to this spinal reflex may be related to the age of the patient when the coarctation was repaired. This probably explains the high incidence of persistent hypertension in patients who undergo coarctation repair after adolescence.