As it was still not possible to elucidate sufficiently the function of the efferent acoustic system or the tractus olivocochlearis (OCB), and as it is still not known whether this system possibly has the function of providing protection against noise-induced trauma, the vestibular nerve was severed in the guinea pig, and the threshold shift was compared with that of a control group. On day 8 after exposure to noise, both the temporary threshold shift (TTS) and the permanent threshold shift (PTS) were significantly lower in the control group than in the group with severed OCB. Thus, it does indeed seem possible to ascribe a protective function to the efferent system.