NMDA receptors within the amygdala play an important role in the acquisition and expression of conditioned fear. Because amygdaloid injections of NMDA receptor antagonists did not block the expression of every behavioral sign of fear, a discussion arose as to whether amygdaloid NMDA receptors play different roles in different kinds of fear-conditioning tasks. To clarify the exact role of amygdaloid NMDA receptors, the present study measured the effects of amygdaloid NMDA receptor blockade on the two major animal models of conditioned fear. An experimental design was used that allowed simultaneous measurement of fear-potentiated startle and freezing during the same test session after animals had undergone identical training procedures. The present study clearly demonstrates that injections of the NMDA receptor antagonist AP-5 into the lateral nucleus of the amygdala significantly attenuated both behavioral fear responses (i.e., the amygdaloid NMDA receptors are necessary for the expression of fear-potentiated startle and freezing). The present results together with others from the literature indicate that NMDA receptors within the lateral amygdala are critically involved in normal synaptic transmission. It appears then that NMDA receptor antagonists may block the acquisition of fear conditioning by directly interfering with normal synaptic transmissions in the amygdala. Possible reasons for some discrepant results in earlier studies are also discussed.