The mechanisms underlying the expression of long-term synaptic potentiation (LTP) in the lateral and medial perforant path inputs to the dentate gyrus were examined in pentobarbital anaesthetized rats. Lateral path LTP was accompanied by a robust, long-lasting reduction in lateral path paired-pulse facilitation (PPF). The reduction in PPF lasted for at least 1 h, and remained even after post-tetanic EPSP slopes were reduced to pre-tetanus levels. The induction of lateral path LTP and the corresponding reduction in PPF were blocked by the competitive NMDA receptor antagonist CPP. Medial path paired-pulse depression was not affected by medial path LTP. These data suggest that changes in transmitter release may contribute more to lateral path than to medial path LTP.