Responses of wide dynamic range neurons in the spinal dorsal horn to input from C fibers with various conduction velocities were analyzed. The wide dynamic range neurons studied were located in the laminae IV-VI of the spinal dorsal horn. The C fiber response to stimulation of the superficial peroneal nerve consisted of three components: early, middle, and late. The separation into three components was found to be caused by asynchronous volleys in three different classes of C fibers in the superficial peroneal nerve. The phenomenon of windup was observed to occur always in the late component, frequently in the middle component, and to a far lesser extent in the early component. The early component was augmented by sciatic nerve compression, indicating that the initial part of the C fiber response is suppressed within the spinal cord by an inhibitory effect of A fiber afferent volleys. An intravenous administration of a specific N-methyl-D-aspartate receptor antagonist, MK-801 hydrogen maleate, significantly suppressed the middle and late components of the C fiber response, although the effect on the early component was insignificant. Thus, one can infer that the hyperexcitability of wide dynamic range neurons to input from early components of cutaneous C fiber stimuli will yield hyperpathic symptoms such as paresthesia, hyperalgesia, and allodynia associated with damaged peripheral nerves, and that learning how to inhibit the hyperexcitability of wide dynamic range neurons to input from early components of C fiber stimuli will lead to the curative treatment for neuropathic pain.