Thresholds were measured for 5-ms 1-kHz tones masked by synchronous bursts of noise containing a spectral notch centered on the signal frequency. Thresholds were reduced by prior exposure to a 200-ms burst of a 'priming stimulus' which had the same spectral shape as the masker. The masking release was greatest for notch widths extending between 20-30% above and below the signal frequency. It did not occur when the masker and primer were bandpass noises extending from 200-1800 Hz. A smaller masking release could be obtained with a primer consisting of only the lower band of a notched noise masker. This was also true, but to a lesser extent, for a primer consisting of the higher band alone. A primer that was a narrow band of noise centered on the signal frequency produced an increase in masking, which could not be attributed to forward masking of the tone by the primer. The effects of all primers were independent of primer level over the 30-dB range studied, ruling out explanations in terms of peripheral adaptation or of adaptation of suppression. A significant masking release occurred when the silent interval between primer offset and masker onset was as long as 320 ms, but increasing the duration of the masker and signal beyond 80 ms eliminated the effect in two out of three subjects. The results are consistent with a form of processing which groups together energy in frequency regions containing common amplitude envelopes, and which enhances the internal representation of newly-arriving energy in previously unstimulated frequency regions.