The attenuation of incoming sounds induced by acoustic reflex triggering was evaluated from the cochlear microphonic response to test tones in 45 awake guinea-pigs. Although control electromyographic measurements proved that the stapedius muscle was contracting, neither impedance changes nor attenuation induced by contralateral reflex-eliciting sounds were detectable in 30 cases out of 45. For ipsi- and bilateral stimulations, an attenuation was detectable for 7 guinea-pigs out of 10. In the guinea-pigs for which a reflex-induced change was found on CM, the mean attenuation was weak i.e. of the order of 2 dB at 20 dB above reflex threshold. These results were quite different from those obtained during control experiments in the rabbit for which CM attenuation was much larger. However, large attenuations associated with middle ear muscle contractions were found in the guinea-pig in other circumstances, i.e. during self-vocalization or when spontaneous muscle contractions occurred during anaesthesia. It is concluded that middle ear muscles can have several different functions, and that even when it exists, attenuation of loud sounds might not be their primary role.