Protection against noise trauma results by sound-conditioning animals to a low-level, long-term acoustic stimulus prior to a damaging exposure. It is known that the outer hair cells are selectively protected by sound-conditioning. The aim of the present study was to determine if the intrinsic properties of the outer hair cell had been modified by sound-conditioning. A stimulus-related increase in the vesicle content in the presynaptic region was found. It is suggested that the outer hair cells are involved in sound conditioning and that changes in the presynaptic region can be one correlate to the protection against noise trauma by sound-conditioning.