Long-term potentiation (LTP) can be induced in the lateral and basolateral amygdala by stimulating synaptic afferents in the external capsule (EC). We examined the sensitivity of amygdaloid LTP to the NMDA receptor antagonist 2-amino-5-phosphonopentanoate (AP5), which is known to block LTP induction in the Schaffer collateral/CA1 synapses in the hippocampus. While relatively high concentrations (100 microM) of DL-AP5 were effective in preventing LTP induction in the lateral and basolateral amygdala in vitro, the same concentrations also significantly depressed synaptic responses to low-frequency stimulation. Furthermore, at 50 microM, a concentration sufficient to block both synaptic responses mediated by NMDA receptors and LTP induction in the hippocampus and neocortex, AP5 did not affect the probability of inducing LTP in the amygdala. Application of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), which blocks non-NMDA excitatory amino acid receptors, reduced the monosynaptic response to EC stimulation by 85%. The remaining CNQX-insensitive response did not appear to be mediated by NMDA-type receptors, since it was not reduced by 50 or 100 microM AP5, and showed none of the voltage sensitivity characteristic of NMDA responses. These data suggest that while the induction of LTP in the amygdala produced by EC stimulation is blocked by high doses of AP5, plasticity at these synapses probably does not require activation of NMDA receptors.