It is widely accepted that N-methyl-D-aspartate (NMDA)-receptor-dependent long-term potentiation (LTP) in the CA1 region of the hippocampus is triggered postsynaptically, but there is considerable debate as to the site at which the increase in synaptic strength is expressed. The irreversible open-channel blocking action of the NMDA receptor antagonist MK-801 has been used to test whether the probability of transmitter release (P-r) is increased during LTP. Although the rate of decline of the amplitude of the NMDA receptor-mediated excitatory postsynaptic current (EPSC) in the presence of MK-801 strongly depends on P-r, the rate of decline of the EPSC evoked at synapses expressing LTP is identical to that observed at synapses not expressing LTP. These findings are difficult to reconcile with models in which the expression of LTP is due to an increase in P-r.