The amplitude of the acoustic startle response habituates to repetitive stimulation. The input and output of the startle system were measured to determine if the decrease in startle amplitude during repetitive stimulation is due to an increase in the startle threshold. Two experimental approaches were used in 35 Sprague-Dawley rats to probe the relationship between the input (the sound pressure level of the stimulus) and the behavioral output (startle amplitude). The results show that the minimum threshold for a response does not change during habituation; rather, the slope of the dependence of startle amplitude on stimulus level decreases. Because habituation does not influence startle threshold we propose that the site for habituation is located in the neural circuitry downstream from the site for startle threshold. Besides amplitude and threshold, as an additional parameter we measured startle latency. In general, the latency of the acoustic startle response is negatively correlated with the response amplitude. This correlation has been repeatedly shown, therefore one would expect a latency increase during the amplitude decrease caused by habituation. However, the latency of the startle reaction also decreased during the course of repetitive stimulation. According to the dual process theory of habituation, a stimulus has both a response-decreasing, i. e., habituating, as well as a response-increasing, i.e., sensitizing, influence on a behavior (Groves & Thompson, 1970). Our explanation of the present results is that startle amplitude is reduced following repetitive stimulation because it is mainly influenced by habituation; latency, however, is shortened because it is mainly influenced by sensitization.