In conditions of facilitated synaptic release, CA3/CA1 synapses generate anomalously slow NMDA receptor-mediated EPSCs (EPSC(NMDA)). Such a time course has been attributed to the cooperation of synapses through glutamate spillover. Imitating a natural pattern of activity, we have applied short bursts (2-7 stimuli) of high-frequency stimulation and observed a spike-to-spike slow-down of the EPSC(NMDA) kinetics, which accompanied synaptic facilitation. It was found that the early component of the EPSC(NMDA) and the burst-induced late component of the EPSC(NMDA) have distinct pharmacological properties. The competitive NMDA antagonist R-(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (D-CPP), which has higher affinity to NR2A than to NR2B subunits and lowest affinity at NR2D subunits, significantly slowed down the decay rate of the afterburst EPSC while leaving the kinetics of the control current unaffected. In contrast, ifenprodil, a highly selective NR2B antagonist, and [+/-]-cis-1-[phenanthren-2yl-carbonyl]piperazine-2,3-dicarboxylic acid (PPDA), a competitive antagonist that is moderately selective for NR2D subunits, more strongly inhibited the late component of the afterburst EPSC(NMDA). The receptors formed by NR2B and (especially) NR2D subunits are known to have higher agonist sensitivity and much slower deactivation kinetics than NR2A-containing receptors. Furthermore, NR2B is preferentially and NR2D is exclusively located on extrasynaptic membranes. As the density of active synapses increases, the confluence of released glutamate makes EPSC decay much longer by activating more extrasynaptic NR2B- and NR2D-subunit-containing receptors. Long-term potentiation (LTP) induced by successive rounds of burst stimulation is accompanied by a long-term increase in the contribution of extrasynaptic receptors in the afterburst EPSC(NMDA.)