Cell bodies in the lateral and basolateral amygdaloid nuclei were destroyed by local infusion of N-methyl-D-aspartate. Adjacent areas, such as the central amygdaloid nucleus, were largely spared. Lesions were carried out before training and testing (Experiment 1) or after training but before testing (Experiment 2). In both cases, the lesions completely blocked fear-potentiated startle (increased acoustic startle in the presence of a light previously paired with footshock). They also blocked increased startle after a series of footshocks, provided they damaged the most anterior part of the basolateral nucleus. It is suggested that the lateral or basolateral amygdaloid nuclei (or both) relay visual information to the central amygdaloid nucleus, which is also critical for fear-potentiated startle. In addition, activation of the most anterior part of the basolateral nucleus may be critical for processing shock information during fear conditioning.