Abstract: The thresholds of sound signal perception in the presence or absence of a continuous background sound were compared on 6 subjects. When both the signal and the background sound were pure tone of the same frequency, the thresholds of signal perception in the presence of weak background sound (0-5 db above threshold) were about 8 db lower, in average, than when the background sound was absent. This lowering of signal perception threshold was explained as a result of sensibilization of the auditory system by the background sound. Such kind of sensibi-lization effect was not eliminated by the masking of white noise. It, however, did not appear, if the signal differed from the background sound in frequency in more than 10 cps.