This study was aimed at investigating the effects of lamotrigine (LTG) on electrically evoked field excitatory postsynaptic potentials (fEPSP) and population spikes in the CA1 hippocampal region of guinea pigs. The concentration response curves showed different actions of LTG on fEPSP and on population spikes. The data are in contrast to previous findings that suggest the drug acts primarily on presynaptic sites via a blockade of the release of excitatory amino acids, In the range of therapeutic plasma levels, synaptic transmission was not affected. Copyright (C) 2000 S. Karger AG, Basel.