THERMORECEPTORS reported in the skin of the cat, rat and dog1–3 had properties similar to the thermoreceptors found in the tongue4. The afferent fibres, however, had low conduction velocities characteristic of non-myelinated axons. The receptors fell into two categories: those excited by a rise and those excited by a fall in cutaneous temperature, above or below the neutral temperature of the skin. At certain temperatures each of the fibres was firing steadily and when tested would continue to respond at a nearly constant rate of discharge for 30 min or longer. The very low conduction velocities of all the afferent fibres, less than 1.5 m/sec, was difficult to fit with results based on psycho-physical and nerve-block techniques in man, which indicated that most of the afferent fibres mediating cutaneous thermal sensationssees. had conduction velocities faster than 4 m/sec5, although Landau and Bishop6 have more recently brought forward evidence that in man, too, some thermal sensations can be mediated by non-myelinated fibres in peripheral nerves.