Results were reported of psychophysical forward-masking experiments using a lateralization method. A general interpretation of masking was given, considering masking to be the combined result of three different mechanisms: the overlap mechanism, the adaptation mechanism, and the suppression mechanism. The aim of this study was to demonstrate the use of the lateralization method in a masking experiment. Masking was measured in a band-widening experiment using a test tone frequency of 3 kHz, which is the center frequency of the masking noise. It was found that the effect of the suppression mechanism depends in a complex way on the difference between masker level and test tone level, as does the bandwidth at which maximum masking occurs. These level effects could be described qualitatively by means of nonlinear excitation patterns.