We investigated the concept of a critical masking interval, the time period during which noise is effective in masking a brief signal. The signal was a 100‐μsec click temporally placed in the center of a gated‐noise masker. Both the click and the noise were low‐pass filtered so that the power spectra of signal and noise have similar shape. The duration of the noise masker was varied from 100 μsec to 300 msec and two noise levels were employed. Like the estimate of the critical bandwidth, the estimate of the critical masking interval can be made in two independent ways. First, for long durations of the noise masker, the ratio of click energy to overall noise power indicates that the critical masking interval is about 14 msec. Second, progressively shortening the duration of the noise masker did not improve detectability until the duration was less than approximately 10 msec. The near agreement of these two independent estimates supports our interpretation of the data in terms of the concept of a critical masking interval.