The amygdala plays an important role in emotional learning. Synaptic plasticity underlying the acquisition of conditioned fear occurs in the lateral nucleus of the amygdala: long-term potentiation (LTP) of synapses in the pathway of the conditioned stimulus (CS) has shown to be a neural correlate of this kind of emotional learning. The present study is based on previous results of our laboratory showing an important role of the metabotropic glutamate receptor subtype 5 (mGluR5) in fear conditioning. Here, we explored whether mGlu5 receptors within the lateral nucleus of the amygdala are involved in the plasticity underlying fear conditioning. We used an in vivo approach investigating the acquisition, consolidation and expression of conditioned fear by the fear-potentiated startle paradigm and by the inhibition of motor activity during CS presentation. Additionally, we used an in vitro approach inducing LTP in the lateral amygdala by patch-clamp recordings in rat brain slices. Acquisition of conditioned fear, but not consolidation and expression, was blocked by intra-amygdaloid injections of the specific mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP) in vivo. Furthermore, induction of amygdaloid LTP but not synaptic transmission was disrupted by MPEP application in vitro. These experiments show for the first time in vivo and in vitro that mGluR5 receptors are necessary for plasticity in the amygdala.