Many fast excitatory synapses in the hippocampus are enriched with both AMPARs (-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate receptors) and NMDARs (N-methyl-D-aspartate receptors). Their proximity allows them to be activated simultaneously by the same neurotransmitter, L-glutamate. Activation of AMPARs leads to influx of sodium and calcium ions, which can increase or decrease NMDAR activity through sodium concentration-dependent cascades or a calcium-calmodulin-dependent inactivation process, respectively. Here we provide evidence that the activation of AMPARs inhibits NMDARs through a non-ionotropic mechanism. NMDA-induced current in isolated CA1 hippocampal cells and nucleated patches of cultured hippocampal neurones decreased when AMPARs were activated. Conversely, when AMPARs were blocked, the NMDA component of glutamate-induced current increased. The inhibitory action of AMPAR activation on NMDAR-mediated current depends upon the open state of AMPA channels and rapidly diminishes after deactivation of AMPARs. The inhibitory action was independent of membrane voltage, univalent cation fluxes and calcium influx. The AMPA-NMDA cross-inhibition also occurred in evoked synaptic current in CA1 neurones from intact hippocampal slices. This cross-talk may play a role in preventing overexcitation during bursting activities in the hippocampus.