In the mammalian brain most excitatory transmission is mediated by glutamate binding to AMPA and NMDA receptors. These receptors have markedly different biophysical properties, and at synapses in the CAI region of the hippocampus they play complementary roles in long-term potentiation (LTP): while postsynaptic NMDA receptor activation is necessary for the induction of this form of plasticity, AMPA receptors play a larger role in its expression. Recent studies in hippocampal slices have revealed a further striking difference in the behaviour of the two receptor types: NMDA receptors consistently sense a larger number of quanta of glutamate released from presynaptic terminals than do AMPA receptors. Two alternative explanations for this are either that AMPA receptors are functionally silent at a proportion of synapses (although they can be uncovered by LTP), or that glutamate can spill over from neighbouring synapses and selectively activate NMDA (but not AMPA) receptors. Both of these competing hypotheses have extensive implications for the mechanisms of expression of LTP. Extrasynaptic glutamate diffusion appears to depend critically on the recording temperature, but if excitatory synapses are sufficiently close for cross-talk to occur under physiological conditions, it could have profound implications for the specificity of synaptic communication in the brain.