The primary object of this investigation has been to detect by physiologic means the fibers in peripheral nerves which conduct the impulses that give rise to painful sensation. Both man and animals have been employed as experimental material. The results have been interpreted in the light of previous investigations of the properties of the different groups of nerve fibers occurring in various nerves, their action potentials being recorded, both in excised preparations and in situ, by means of the cathode-ray oscillograph. Secondarily, we have been interested in the differences in quality between the sensation resulting from stimuli applied to cutaneous nerve organs and that from stimuli applied to nerve trunks directly. We also believe our data support the hypothesis that nerve fibers occur in groups, not only according to size and conduction rate, but also according to their terminal connections, such that a group of fibers which have similar properties