Activity-dependent enhancement of synaptic transmission, referred to as long-term potentiation (LTP), is observed at many synapses in the central nervous system. In the hippocampus two distinct forms of LTP have been identified. One involves the activation of the NMDA (N-methyl-D-aspartate) subtype of glutamate receptor and a rise in postsynaptic Ca2+, whereas the other, which is found at mossy fibre synapses, is independent of NMDA receptors but does require a rise in presynaptic Ca2+. Although it is now generally accepted that mossy fibre LTP is expressed presynaptically, the locus of expression for NMDA-receptor-dependent LTP is controversial. Here the two forms of LTP are compared and it is argued that the balance of evidence favours a postsynaptic locus for NMDA-receptor-dependent LTP.