Lesions of the amygdala have been shown to block the expression of fear-potentiated startle (increased acoustic startle in the presence of a cue previously paired with shock). In the present study, bilateral lesions of the central nucleus of the amygdala given after extensive training totally blocked the expression of fear-potentiated startle but did not prevent reacquisition. In contrast, when the lesions were made before any training, the lesioned rats did not show potentiated startle even with extensive training. Thus, the central nucleus of the amygdala normally seems to be required for the initial acquisition and expression of potentiated startle regardless of the degree of learning. However, reacquisition of potentiated startle can occur without the central nucleus, which implies the presence of a secondary brain system that can compensate for the loss of the central nucleus of the amygdala under some circumstances.