Coarctation of the aorta presents several features of interest concerning the hemodynamics and regulation of the circulation, for it presents a condition in which the brain and carotid sinus, the chief sites of any central regulatory functions, are subjected to an arterial hypertension while the lower extremities and viscera are supplied with blood at a lower pressure. The cardiac output has not hitherto been studied in cases of coarctation. Because of the relative infrequency with which the condition is recognized in young persons before the advent of secondary complications, and because of the interesting theoretical considerations involved, the results obtained in a single person have been considered worthy of a report. REPORT OF A CASE Clinical History. —The subject of our study, J. P. F., Jr., was a medical student, 25 years of age. His family and past history were entirely irrelevant. Except for diphtheria, measles, chickenpox and occasional colds,