Afferent nerve activity from left and right atrial, left and right ventricular, interventricular septal, papillary muscle, pericardial, aortic and pulmonary vascular receptors was recorded from the left T3 white ramus communicans and the innominate, dorsal, recurrent, ventromedial, craniovagal and caudovagal cardiac nerves in alpha-chloralose-anesthetized dogs. The receptors were localized and the nature of the stimuli required to excite these receptors was also determined. Some ventricular receptors were excited during maximal contraction of the myocardium. Other ventricular and aortic receptors were stimulated by elevation of intracardiac or aortic pressure. The pericardial, atrial, and papillary muscle receptors were excited by stretching the surrounding tissue. The discharge patterns of these receptors were not always synchronous with the events of the cardiac cycle. The stimuli required to excite each type of receptor (ventricular, atrial, etc.) and their resultant discharge patterns were not identical for all of the receptors. Excitation of cardiac receptors with sympathetic afferents resulted in 1--2 spikes per cardiac cycle, whereas receptors with vagal afferents resulted in bursts of spikes per cardiac cycle.