We have previously found that the induction of hippocampal long-term potentiation (LTP) is modulated by neuron activities in the basolateral amygdala (BLA). However, little is known about what neurotransmitter system in the BLA contributes to modulation of hippocampal LTP. In the present study, we investigated possible involvement of BLA serotonergic system in the induction of LTP at the perforant path (PP)-dentate gyrus (DG) granule cell synapses of anesthetized rats. The induction of PP-DG LTP was significantly inhibited by intra-BLA injection of the 5-HT(2) receptor antagonist cinanserin (25-50nmol), but not by intra-BLA injection of the 5-HT(1,7) receptor antagonist methiothepin (50nmol), the 5-HT(3) receptor antagonist ondansetron (50nmol) or the 5-HT(4) receptor antagonist RS23597-190 (100nmol). In addition, intra-BLA injection of the 5-HT(2C) receptor agonist MK212 (50nmol) facilitated the induction of PP-DG LTP. These results suggest that the induction of PP-DG LTP is promoted by activation of 5-HT(2C) receptors in the BLA.