The results from studies on selective adaptation to amplitude modulation (AM) have been interpreted as providing support for the existence of neural channels specialized for processing of AM. These studies have used long exposure times (of the order of minutes) and relatively long durations of the test modulation (typically a few seconds). In contrast, the present study demonstrates a strong effect of prior exposure to AM after exposures shorter than 200 ms. This is analogous to forward masking (in the audio frequency domain) and appears to reflect a fundamentally different process from that underlying AM adaptation. More specifically, a 150-ms, 100%-modulated AM masker substantially impairs detection of a 50-ms burst of ensuing modulation imposed on the same carrier. Thresholds for detecting the brief signal modulation are elevated for temporal separations between the masker and signal AM up to about 200 ms but not for longer separations. Our data also show broad tuning of this AM forward-masking effect. Control conditions using cueing stimuli that mark the temporal end of the masker modulation, indicate that the forward masking effect is not due to temporal confusion. The results strongly suggest that there is sensory basis for the observed effect, possibly involving modulation-rate selective neural channels that adapt or continue being activated beyond the offset of the masker modulation. Implications of this surprising phenomenon for the processing of complex stimuli with fluctuating envelopes will be discussed. [Work supported by Grant No. DC00683 from NIDCD].