The activation of N-methyl-D-aspartate receptors is implicated in the spinal and trigeminal processing of nociceptive information conveyed by convergent (wide dynamic range) neurons and particularly in C-¢ber-evoked responses elicited by repetitive and high-intensity electrical stimulation of the neuronal receptive ¢eld. In this study, the e¡ects of intrathecal NMDA application on the electrically evoked nociceptive responses of trigeminal subnucleus caudalis convergent neurons have been investigated. The total C-¢ber-evoked activity triggered by 30 successive stimuli was divided into two components: the C-¢ber input response and thèwind-up' response. Application of 0.1 Wg (in 50 Wl) of NMDA evoked a bi-directional e¡ect on the total C-¢ber-evoked activity of 19 neurons tested. A signi¢cant increase in the total C-¢ber-evoked activity was observed 15^25 min after the NMDA application for nine neurons located in super¢cial laminae II and III. In contrast, a signi¢cant decrease in the total C-¢ber-evoked activity was observed 5^25 min after the NMDA application for 10 neurons located more deeply, in lamina V. The NMDA-induced modi¢cations seen in the total C-¢ber-evoked activity were likely a re£ection of a signi¢cant increase or decrease in neuronal activity evoked by the C-¢ber input rather than wind-up of the responses since the latter was not signi¢cantly modi¢ed by the NMDA application. These results provide evidence for a possible inhibitory role for NMDA-dependent interneurons of the super¢cial laminae of the medullary dorsal horn on the nociceptive activity of deep convergent neurons. ß