Application of an intense stimulus to the skin can result in the perception of a sharp pain followed by that of a dull, burning pain. These two distinct sensations have been referred to as early and late pain or first and second pain, and they are due to impulses arriving at the cord in cutaneous A and C fibers, respectively. Price (3) has compared the characteristics of first and second pain in human subjects to those of flexion reflexes in the cat. The early component of the EMG discharge of flexor muscles following cutaneous stimulation was attributed, like first pain, to activity in A fibers, whereas second pain and the late discharges were considered to be due to C fibers. Repetition of the strong stimulus (above the threshold for C fibers) resulted in a progressive increase in the second pain and in the late component of the flexor reflex (sensitization). The early component of the reflex remained constant and the sensation of first pain either decreased or did not change. Sensitization of the late component of the reflex and a second pain have been attributed to the behavior of some interneurons of the dorsal horn of the spinal cord (3). The response of such cells to stimulation of cutaneous C fibers occurs after a long latency (100-200 msec) and consists of a prolonged train of action potentials which often lasts for several hundreds of milliseconds. The duration of this discharge increases with each successive stimulus (I, 4). Thus, experiments on the cat indicate that only the late component of the flexor reflex undergoes sensitization and that this