The amygdala plays a critical role in the mediation of emotional responses, particularly fear, in both humans and animals(1-4). Fear conditioning, a conditioned learning paradigm, has served as a model for emotional learning in animals, and the neuroanatomical circuitry underlying the auditory fear-conditioning paradigm is well characterized(5). Synaptic transmission in the medial geniculate nucleus (MGN) to lateral nucleus of the amygdala (LA) pathway, a key segment of the auditory fear conditioning circuit, is mediated largely through N-methyl-D-aspartate (NMDA) and non-NMDA (such as alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)) glutamate receptors(6); the potential for neural plasticity in this pathway is suggested by its capacity to support long-term potentiation (LTP)(7,8). Here we report a long-lasting increase in the synaptic efficacy of the MGN-LA pathway attributable to fear-conditioning itself, rather than an electrically induced model of learning. Fear-conditioned animals show a presynaptic facilitation of AMPA-receptor-mediated transmission, directly measured in vitro with whole-cell recordings in lateral amygdala neurons. These findings represent one of the first in vitro measures of synaptic plasticity resulting from emotional learning by whole animals.