Glutamate has traditionally been regarded as an excitatory neurotransmitter. Synaptic activation of ionotropic glutamate receptors mediates fast EPSPs in the CNS. Moreover, activation of metabotropic glutamate receptors (mGluRs), which are coupled to second messenger effector systems via GTP-binding proteins (G-proteins), results in the expression of slow EPSPs. We have now examined the response of basolateral amygdala (BLA) neurons to activation of postsynaptic mGluRs. In approximately 78% of BLA neurons examined, activation of postsynaptic mGluRs results in membrane hyperpolarization and an associated decrease in membrane input resistance or a hyperpolarization followed by a depolarization associated with an increase in input resistance. The purpose of this study was to address the mechanisms underlying the membrane hyperpolarization. Here, we report that the ACPD-induced hyperpolarization is insensitive to TTX, is dependent on extracellular K+ concentrations, and has a reversal potential (-84 mV) close to that estimated from the Nernst equation for an increase in a K+ conductance. In addition, the ACPD response is resistant to (1) intracellular chloride loading, (2) the GABAB receptor antagonist CGP55845A, (3) the ACh receptor antagonist atropine, and (4) the ionotropic glutamate receptor antagonists CNQX and APV. These data suggest that the hyperpolarization results from a direct activation of postsynaptic mGluRs on neurons of the BLA. Furthermore, we performed studies that suggest that the hyperpolarization is G-protein mediated and results from activation of a TEA-sensitive, calcium-dependent potassium conductance. The sensitivity of this conductance to thapsigargin further suggests that this response requires the release of calcium from intracellular stores. In summary, these data suggest a role for glutamate as an inhibitory transmitter in the BLA during periods of metabotropic glutamate receptor activation. In nuclei such as the BLA that are exquisitely sensitive to seizure induction, an inhibitory response to glutamate may act to delay the onset of epileptogenesis.