The direct, electrical stimulation of the thoracic anterior roots, the communicating rami, and the upper thoracic sympathetic trunk in the dog reveals the preganglionic pathways followed by the cardiac accelerator and augmentor nerves. These nerve fibers are generally intermingled in each of the above pathways, but accelerator fibers are much more prominent on the right whereas augmentor fibers predominate on the left. The most significant pathways are via the T2 and T3 anterior roots and communicating rami, but significant responses were elicited from the T4 nerves and occasionally from the T1 and T5 nerves. Neither accelerator nor augmentor responses could be induced by stimulation of the C8 nerve nor by excitation of nerves below T5. It is concluded that preganglionic cardiomotor fibers enter the trunk between T1 and T5, course cephalad to the stellate ganglion where they may synapse or pass to the caudal cervical ganglion by way of the ansa subclavia. The stellate cardiac nerve furnishes a very important postganglionic augmentor and accelerator pathway, when it is present, but direct branches from the caudal cervical ganglia carry a majority of these fibers in most animals. Augmentor and accelerator fibers can not be functionally separated at any point in their anatomical pathways from the spinal cord to the heart. Responses seem to be determined rather, by the site of nerve terminations in heart tissue.