The involvement of non-N-methyl-D-aspartate receptors in the amygdala in the expression of conditioned fear was examined using the fear-potentiated startle paradigm. Rats implanted with bilateral cannulae in the basolateral amygdaloid nuclei received 10 pairings of either a visual or auditory conditioned stimulus with footshock on each of 2 days. The next day, they were tested by eliciting the acoustic startle reflex in the presence or absence of the conditioned stimulus and divided into groups with equivalent levels of potentiation. One or two days later, rats were tested again following intra-amygdala infusion of vehicle or 0.025, 0.25, or 2.5 micrograms of 6-cyano-7-nitroquinoxaline-2,3-dione. The drug dose-dependently blocked the expression of potentiated startle in both sensory modalities, indicating that activation of non-NMDA receptors in the amygdala is necessary for the expression of conditioned fear.