We have previously shown that glycine binding sites on presynaptic NMDA receptors (NMDA-Rs) can tonically regulate glutamate release in the rat visual cortex. In the present study, we investigated the subunit composition of these presynaptic NMDA-Rs. We recorded miniature a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated excitatory postsynaptic currents (mEPSCs) using whole-cell voltage clamp in layer II/III pyramidal neurons of the rat visual cortex with the open-channel NMDA receptor blocker, MK-801, in the recording pipette. We found that the frequency of mEPSCs is significantly reduced by 7-chloro-kynurenic acid (7-Cl KYNA) an NMDA-R glycine binding site antagonist, and glycine reverses this effect. Using a specific antagonist for NR2B-NMDA-Rs, Ro 25-6981 [(alphaR,betaS)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidinepropanol hydrochloride], instead of 7-Cl KYNA, we found that the frequency of mEPSCs is also significantly reduced but glycine cannot reverse this effect. Moreover, Zn(2+), an NR2A-NMDA-R antagonist, did not affect mEPSC frequency. These results suggest that presynaptic NR2B-containing NMDA-Rs are located in layer II/III pyramidal neurons of the rat visual cortex, and that the glycine binding site of these type NMDA-Rs tonically regulates glutamate release.