The basolateral amygdala (BLA) is a critical component of the amygdaloid circuit, which is thought to be involved in fear conditioned responses. Using whole-cell patch-clamp recording, we found that activation of nicotinic acetylcholine receptors (nAChRs) leads to an action potential-dependent increase in the frequency of spontaneous GABAergic currents in principal neurons in the BLA. These spontaneous GABAergic currents were abolished by a low Ca²⁺/high Mg²⁺ bathing solution, suggesting that they are spontaneous inhibitory postsynaptic currents (sIPSCs). Blockade of ionotropic glutamate receptors did not prevent this increased frequency of sIPSCs, nor did blockade of ₇ nAChRs. Among the nAChR agonists tested, cystisine was more effective at increasing the frequency of the sIPSCs than nicotine or 1,1-dimethyl-4-phenyl piperazinium iodide (DMPP), consistent with a major contribution of ₄ nAChR subunits. The nicotinic antagonist, dihydro--erythroidine, was less effective than d-tubocurarine in blocking increased sIPSC frequency induced by ACh, suggesting that ₄-containing nAChR subunits do not play a major role in the ACh-induced increase in sIPSC frequency. Although _2/3/4/7 and _2/4 nAChR subunits were found in the BLA by RT-PCR, the agonist and antagonist profiles suggest that ACh-induced increase in sIPSC frequency involves predominantly ₃₄-containing nAChR subunits. Consistent with this, -conotoxin-AuIB, a nAChR antagonist selective for the ₃₄ subunit combination, inhibited ACh-induced increase in the frquency of sIPSCs. The observations suggest that nicotinic activation increases the frequency of sIPSCs in the BLA by acting mainly on ₃₄-containing nicotinic receptors on GABAergic neurons and may play an important role in the modulation of synaptic transmission in the amygdala.