The aim of this work was obtaining quantitative data relative to the non-myelinated fibers of some thoracic nerves of the cat. The study has been done both through the utilization of the light and the electron microscope. A comparative analysis of the results obtained from the muscle nerves (external intercostal nerve and phrenic nerve), cutaneous nerve (lateral collateral branch of the internal intercostal nerve) and mixed nerves (internal intercostal) has been made. The C muscle fibers have a smaller diameter than the C cutaneous fibers. Some of the non-myelinated fibers of the phrenic nerve are probably involved in the innervation of the pleura or of the peritoneum. The relative number of non-myelinated fibers varies according to the nerves. The factor linking the diameter of the non-myelinated fibers to their conduction velocity is approximately equal to 0.7.