The effect of 8-OH-DPAT on the induction of long-term potentiation (LTP) in the lateral nucleus of the amygdala was investigated using rat horizontal brain slice preparations. Bath-applied 8-OH-DPAT decreased the field potential amplitudes in a dose-dependent manner. In the lateral amygdala synapses, 8-OH-DPAT significantly suppressed the induction of LTP evoked by a weak theta burst stimulation. This suppression of LTP was also found using a concentration of 8-OH-DPAT, which did not influence the baseline activity significantly. The specific 5-HT(1A) receptor antagonist, WAY 100,635 blocked the inhibitory effect of 8-OH-DPAT on the induction of LTP. The inhibitory effect of 5-HT(1A) receptor stimulation on amygdaloid neuronal plasticity suggests that the amygdala is a site for serotonin to exert its influence on memory of aversive events.