Anaesthetized guinea pigs were exposed to loud tones (1 h, 10 kHz, 112-118 dB SPL) with continuous control of the sound pressure at the tympanic membrane. N1 electrocochleograms were used to measure functional damage immediately and 21 days after the exposure. Damage to the organ of Corti was assessed by scanning electron microscopy and light and transmission electron microscopy. Principal findings were: (1) Functional impairment after 21 days showed large inter-animal variation which was not the result of changes in the effective damaging energy. (2) Structural damage to the stereocilia was also variable and did not always correlate with functional impairment, although when N1 thresholds were elevated damage to the stereocilia was always present. (3) Unknown factors within the cochlea must be responsible for variations in individual susceptibility to permanent noise-induced hearing loss.