Taurine is an endogenous brain substance with robust neuromodulatory and possible neuroprotective properties. Though other mechanisms of action have been reported, its interaction with the NMDA (N-methyl-d-aspartic acid) receptor is undocumented. We investigated taurine׳s interaction with the NMDA receptor using electrophysiological and receptor binding approaches. The effects of taurine on field potential responses in layer-5 of prelimbic cortex in rat brain slices evoked by single-pulse electrical stimulation of ventral medial cortex were determined. Picrotoxin (80µM) was present in all control and drug solutions to block the Cl− channels associated with the GABA-, taurine-, and strychnine sensitive glycine- receptors. A typical response consisted of an NBQX (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo-[f]-quinoxaline-7-sulfonamide)-sensitive negative wave (N1) followed by a positive wave (P1) and a broad negativity (N2), both sensitive to dl-AP5 (dl-2-amino-5-phosphonopentanoic acid) inhibition. Taurine exerted a 41.5±8.3% (n=9) voltage reduction within the late phase of N2. This taurine action was prevented by 100µM AP5, but not by 10µM nifedipine, supporting a direct modulation of NMDA receptor function by taurine, without requiring the involvement of the l-type Ca2+ channel. Taurine did not alter specific [3H] MK-801 binding to rat cortical membranes in the presence of glycine or glutamate; but inhibited spermine-potentiated specific [3H] MK-801 binding to NMDA receptors by 15–20% in the presence of glycine. In addition, taurine reduced the apparent affinity of the NMDA receptor for glycine (in the presence of spermine) by 10-fold. These results show that taurine interacts directly with the NMDA receptor by multiple mechanisms.