The acoustic startle reflex offers a number of advantages for analyzing the neural bases of behavior. Startle apparently is mediated by excitatory amino acids at several of the synapses that comprise the short-latency startle pathway. The reflex is modulated by a variety of neurotransmitters at both the spinal and the supraspinal levels. In addition to showing habituation and sensitization, startle is increased in the presence of a fear stimulus. This may result from activation of the central nucleus of the amygdala which projects directly tot he acoustic startle pathway. A major challenge for future studies will be to determine what neurotransmitters, which are known to modulate startle, are involved in habituation, sensitization, and fear conditioning and to begin to analyze these processes at a cellular level.