Sodium valproate (VPA) is currently one of the major antiepileptic drugs and has been reported to impair the induction of long-term potentiation (LTP) in rat hippocampal. However, there are discrepancies when used at therapeutic dose and few researches to Study the effects of VPA oil the maintenance of LTP. Here we investigated the effects of VPA at therapeutic concentration oil two LTP phases: induction and maintenance in CA1 region. We found (1) VPA inhibited held excitatory postsynaptic potentials (fEPSPs) without modifying paired-pulse facilitation (PPF) solely occured presynaptically as a form of synaptic plasticity. (2) Pretreatment with VPA before high-frequency stimulation (HFS) decreased the fEPSPs slope. (3) There were no significant changes in fEPSPs slope with VPA applied in maintenance phase of LTP. These results indicate that VPA inhibited the induction of LTP postsynaptically without modifying presynaptic neurotransmitter release and had no significant influence on the two maintenance phase of LTP. (C) 2009 Elsevier Inc. All rights reserved.