Abstract The participation of N- methyl- d-aspartate (NMDA) receptors in synaptic transmission before and after induction of long-term potentiation (LTP) was studied in field CA1 of hippocampal slices. NMDA receptor-mediated postsynaptic responses were determined by comparing responses recorded in the presence and absence of the selective antagonist, d-a-amino-5-phosphonopentanoate ( d-AP5, 50 μM). In the presence of physiological magnesium concentrations (1 mM), robust d-AP5-sensitive responses could be evoked by high frequency bursts (4 pulses, 100 Hz) when burst stimulation was preceded 200 ms earlier by ‘priming’ stimulation (2 pulses, 15 ms apart) of a separate input. Induction of LTP resulted in a substantial potentiation (35%) of non-NMDA-mediated responses to primed bursts but not of NMDA-mediated responses. these results suggest that long-term postsynaptic modifications are at least partly responsible for the expression of LTP.