Synaptic scaling has been reported as scaling up of AMPA receptors (AMPAR)-mediated miniature excitatory postsynaptic currents (mEPSCs) induced by blockade of action potentials or AMPAR. Here, we show a novel type of synaptic scaling induced by N-methyl-D-aspartate receptors (NMDAR) blockade. In the present study, we analyzed AMPAR-mediated mEPSCs of D-(-)-2-amino-5-phosphonopentanoic acid (AP5)-treated hippocampal neurons (16 days in vitro) for 48 h in low-density cultures, using a whole-cell patch-clamp technique. The mEPSC amplitudes recorded from chronic AP5-treated neurons (25.5+/-0.3 pA; n=30 neurons) were significantly larger than that recorded from control neurons (21.6+/-0.2 pA; n=30 neurons, p<0.05), whereas the frequency of mEPSCs was not changed. Immunocytochemistry showed that the number of synapsin I clusters of AP5-treated neurons was not different from that of control neurons. Cumulative amplitude histograms revealed that the amplitude of mEPSCs was scaled multiplicatively after AP5 treatment. GluR2-lacking AMPAR were not involved in the scaling observed here. Together, our data indicate that NMDAR activity, as well as AMPAR activity, is involved in the negative feedback plasticity of AMPAR-mediated synaptic activity.