The minimum detectable duration of a brief burst of noise and a brief gap in a noise were measured by the two-alternative forced-choice method. For all listeners the minimum detectable duration of a burst was shorter than the minimum detectable duration of a gap at equal signal-to-noise ratios. For 12 listeners with normal hearing, the average minimum detectable duration of a burst with a 10-dB signal-to-noise ratio was 1.2 msec, whereas the average of a gap at the same signal-to-noise ratio was 4.2 msec. Nine of ten hearing impaired listeners required longer durations, whether for bursts or gaps, than the average of normal listeners. These nine impaired listeners had slower time constants than any of the normal listeners. Both the rise and decay of auditory sensation in the impaired listeners were therefore abnormally slow.