While cardiac afferent nerve activity has been recorded from the ventrolateral (VLCN) and ventromedial (VMCN) cardiac nerves, left dorsal and ventral ansae subclaviae, and left upper thoracic white rami communicantes, little anatomical evidence for the existence of afferent fibers in these nerves has been reported. This study was designed to characterize the normal ultrastructure of the above nerves and to identify afferent fibers in them through Wallerian degeneration produced by dorsal root ganglionectomy. Laminectomies followed by dorsal root ganglionectomies were performed on left thoracic roots T1-T4 in six mongrel dogs. The nerves to be examined were removed from two animals at 1, 2, and 3 weeks following ganglionectomy and prepared for electron microscopy. Control nerves were obtained from two normal dogs. Degenerating nonmyelinated fibers were characterized by watery axoplasm containing clumps of electron-dense material. Degenerating myelinated fibers were distinguished by the separation of their myelin lamellae, producing characteristic whorls. After three weeks, afferent nonmyelinated axons had degenerated in all nerves, leaving only layered processes of Schwann cells in these areas. Approximately 5-15% of the fibers in each nerve degenerated, indicating their afferent nature. Of these fibers, 85-90% were nonmyelinated C fibers and the remainder myelinated Adelta fibers. These results indicate participation of both Adelta fibers and a large population of C fibers in transmission of cardiac afferent activity.