The masking phenomenon is used in clinical investigation of the inner ear to determine the frequency selectivity properties of the auditory system. Sectioning of the crossed part of the medial efferent bundle in a guinea pig model decreases the simultaneous masking phenomenon. The efferent effect seen in the forward masking varied with the masking paradigm used. When the masker onset precedes the maskee onset by more than 40 ms, masking diminishes after sectioning the crossed part of the medial efferent bundle. When this duration was shortest (30 ms), no effect was observed. This phenomenon could be explained by the time necessary to stimulate the medial efferent loop. Our results could explain some differences observed in the compound action potential masking curves with different masking paradigms.