The present research used a startle amplitude reduction paradigm to investigate the ability of the rat's auditory system to track rapidly changing acoustic transients. Specifically examined was the ability of brief gaps in otherwise continuous noise to reduce the amplitude of a subsequently elicited acoustic startle reflex. The duration of the gap, time between gap offset and startle elicitation (the interstimulus interval or lSI), and rise-fall characteristics of the gap were systematically varied. Consistent with previous research, gaps reliably reduced startle amplitude. Gaps 2 msec long were reliably detected, and a 50-msec lSI resulted in the greatest amplitude reduction. Gaps presented at short ISIs produced amplitude reduction that followed a different time course than did gaps presented at longer ISIs. These results may reflect differences in the length of time available for the processing of the stimulus and may involve two different processes.