Damage to the chick cochlea in response to progressively increased periods of noise exposure was studied with scanning electron microscopy. Ten day-old chick hatchlings were exposed to a 1500 Hz pure tone at 120 dB SPL for 4, 8, 12, 24, and 48 h. Measurements of hair cell and supporting cell surface areas within a defined region of the cochlea showed that the average hair cell surface area decreased over the first 12 h of exposure. Between 12 h and 48 h there was no significant change in hair cell surface area. Supporting cells showed a corresponding increase in surface area over the same period. Noise damage first appeared after 4 h of exposure as a localized expansion of supporting cell surfaces near the inferior edge of the basilar papilla (BP). Between 8 and 12 h of exposure the supporting cell surface area increased dramatically and was visible throughout the noise damaged region. Hair cell expulsion was first seen after 12 h. Exposure to noise for 24-48 h resulted in further expansion of supporting cells, extensive expulsion of hair cells from the BP, and the appearance of a strip of noise damage along the superior, edge of the BP.