The mammalian efferent medial olivo-cochlear system modulates active amplification of low-level sounds in the cochlea. Changes of the cochlear amplifier can be monitored by distortion product otoacoustic emissions (DPOAEs). The quadratic distortion product f2-f1 is known to be sensitive to changes in the operating point of the amplifier transfer function. We investigated the effect of contralateral acoustical stimulation (CAS), known to elicit efferent activity, on DPOAEs in the gerbil. During CAS, a significant increase of the f2-f1 level occurred already at low contralateral noise levels (20 dB SPL), while 2f1-f2 was much less affected. The effect strength depended on the CAS level and as shown in experiments with pure-tones on the the frequency of the contralateral stimulus. In a second approach, we biased the position of the cochlear partition and hence the cochlear amplifier operating point periodically by a ipsilateral low frequency tone, which resulted in a phase-related amplitude modulation of f2-f1. This modulation pattern was changed pronouncedly during contralateral noise stimulation, in dependence of the noise level. The experimental results were in good agreement with a simple model of distortion product generation, and suggest that the olivo-cochlear efferents might change the operating state of cochlear amplification.