THE manner in which the cardiovascular system responds to the increased metabolic demands of muscular exercise is of considerable interest to the clinician and physiologist. It has been suggested that the sympathetic nervous system plays an important part in mediating this response in normal man.^{1 2 3 4 5} In patients in whom the cardiac reserve is diminished, the normal increase of cardiac output during exercise is attenuated, or even abolished.^{6 , 7} It is not known to what extent the sympathetic nervous system is called into play under these circumstances. Diminished activity of this important reserve mechanism could be a factor in the abnormal cardiac . . .