The mechanical activation of localized regions of the vena cava, atria, ventricles, or thoracic aorta reflexly initiate preganglionic and postganglionic efferent sympathetic traffic in cardiac nerves. This traffic responds to altered cardio-dynamics. On the other hand, efferent vagal traffic in cardiac nerves is directly modified only by carotid and pulmonary artery mechanics. Afferent stimulation of cardiac nerves elicits reflex activation of preganglionic and postganglionic cardiac sympathetic efferents from 20 to 300 msec after the stimulus. These cardio-cardiac reflexes involve afferents travelling via the sympathetics or vagi. The modulation of cardiac parasympathetic or sympathetic efferent traffic throughout each cardiac cycle indicates not only the highly localized nature of the cardio-cardiac and vascular-cardiac reflexes, but also their rapid adaptability, and the presence of positive as well as negative feedback mechanisms.