Response curves (in terms of percent of positive responses) to intensity variations at levels of 10, 20, or 40 db above threshold while stimulating the opposite ear with a continuous tone of the same frequency (1000 cps) at a level of 10, 20, 40, or 80 db above threshold, were studied on three subjects. In all cases, a contralateral continuous tone, although of same frequency, provokes always an increase of the differential threshold in the tested ear. The differential threshold is directly related to the intensity applied on the opposite ear, but inversely related to the intensity applied on the tested ear. By combining, for the same subjects, the values of undisturbed thresholds and of those disturbed by a contralateral tone, we can translate results into equivalent levels and to equivalent values of attenuation. The equivalent level decreases rapidly, and the decrease is steeper as the sound intensity on the tested ear is increased. The experiments suggest that the effect described above is not due to masking. The author proposes the idea that a sufficient contrast relative to the constant background (resulting from the tone applied on the contralateral ear) is necessary for perceiving differences of intensity.