We investigated whether N-methyl-D-aspartate (NMDA)-induced slow current in substantia gelatinosa (SG) neurons might be of presynaptic or postsynaptic origin. Whole-cell voltage-clamp recordings were made from adult rat spinal cord slices to examine the effect of NMDA receptor upon miniature excitatory postsynaptic currents (mEPSCs) in SG neurons. At a holding potential of -70 mV, bath application of NMDA (10 microM, 30s), an NMDA receptor agonist, affected neither the frequency nor the mean amplitude of mEPSCs, while NMDA induced a slow excitatory membrane current which persisted in the presence of tetrodotoxin. On the other hand, AP-5 (an NMDA receptor antagonist) had no noticeable effects on mEPSCs in the SG neurons tested, while it markedly depressed the NMDA-induced slow currents. The NMDA-induced slow currents varied with the changing of holding potentials (from -70 to 0 mV). The results suggested that the NMDA-induced slow currents in SG neurons were of postsynaptic origin.