Psychophysical tuning curves obtained in forward masking show greater tuning than those obtained in simultaneous masking. This difference is often attributed to the contribution of suppression to the masking produced by off-frequency simultaneous maskers. In this experiment, simultaneous-masking tuning curves were obtained using a 195-ms, 1-kHz sinusoidal signal presented at 40 dB SPL. If the maskers identified in this procedure reduce signal detectability by suppressing the response to the signal, then it should be possible to demonstrate suppression effects between stimuli with the parameters of the masker and signal. One conventional method for demonstrating suppression is to show a reduction in the amount of forward masking produced by one stimulus when a second stimulus is added to it. When used to test the effect of the masker upon the signal, this procedure does not show the suppression supposedly produced by the off-frequency maskers. These data are consistent with an alternative explanation involving only excitatory interactions between masker and signal.