Compound Action Potential Tuning Curves (CAP-TC) for tone pip of 2k, 4 kHz were examined in 8 guinea pigs before and after exposure to noise with main energy centered in the range of 0.25-4.0 kHz. CAP-TC was measured with the pure tone simultaneous masking profiles. AP was evoked by tone pip with an intensity of 10 dB above threshold. Masker level producing 40% reduction in AP amplitude was used. Relations between changes in CAP-TC and AP threshold shifts and the pathology of the stereocilia of hair cells were evaluated by surface preparation and SEM observation in 13 ears. After noise exposure, animals with damaged stereocilia showed AP threshold shift of 20-50 dB, deteriorations of CAP-TC, decrease of Q10 dB value, threshold shift of characteristic frequencies (CF) and displacement of CF towards higher frequencies. It showed that stereocilia damage may affect the susceptibility and frequency selectivity of the cochlea. We consider the CAP-TC may be an useful and sensitive index for detecting physiological and pathological conditions of the cochlea.