The locations of sympathetic postganglionic and parasympathetic preganglionic neurons projecting to the heart have not yet been clearly established. Therefore, aliquots of horseradish peroxidase (HRP) were injected into specific regions of the heart, intraventricular cavity, pericardial sac, aortic arch, and the skin in 27 dogs. Following injections into the heart, aorta, or pericardial sac, retrogradely labeled neurons were present in the greatest numbers in the middle cervical ganglia bilaterally. Labeled neurons were located in the cranial poles of the stellate ganglia bilaterally and occasionally in the superior cervical ganglia. Labeled cells were also found in small ganglia located along cardiopulmonary nerves. Injections of HRP into specific areas of the heart did not result in labeling of cells in specific loci of the thoracic or cervical sympathetic ganglia. When the ansae were cut on one side, no labeled cells were found in the ipsilateral stellate ganglion or upper thoracic chain. Following injections into the skin of the left elbow or left cranial nipple, labeled cells were found in the stellate ganglia and the sympathetic chain and in one case in the middle cervical ganglion as well. These data suggest that postganglionic sympathetic neurons which project efferent axons to a specific cardiac region are not located in a specific region of a sympathetic ganglion or a specific sympathetic ganglion. Rather, neurons in one region of a sympathetic ganglion project axons to widespread areas of the myocardium. Sympathetic postganglionic neurons in the stellate ganglion or sympathetic chain project their axons to the heart via the subclavian ansae or interganglionic nerves, not via nerves arising directly from the sympathetic chain. Small numbers of labeled neurons were found in the medulla oblongata, thus indicating that in comparison to sympathetic postganglionic neurons relatively few preganglionic parasympathetic neurons project directly to the heart. When labeled cells were present in the medulla, the majority were located in the ventrolateral nucleus ambiguus.