Coarctation of the aorta or stenosis of the aortic isthmus is of considerable interest, not only because of its rarity, but also because arterial hypertension is practically always present. Arterial hypertension exists above the level of the stenosis in this condition in the absence of renal lesions, arteriosclerosis or other complicating factors. The blood pressure below the level of the stenosis is much lower. Coarctation of the aorta, therefore, presents an unusual opportunity to compare in the same person the dynamics of the circulation in arterial hypertension as it exists above the coarctation with the dynamics of blood flow under lower pressure below the level of the stenosis. Coarctation of the aorta has been diagnosed clinically in almost eighty patients, but no studies of the blood flow have hitherto been recorded. The purpose of this communication is to present the clinical observations in two patients with coarctation of the aorta,