When chinchillas are continuously exposed for more than 24 h to an octave band of noise centered at 500 Hz, the behavioral TTS for 715‐Hz tone stabilizes at an asymptotic value proportional to band level [Carder and Miller, Trans. Amer. Acad. Ophthal. Otolaryng.75, 1346 (1971)]. Recovery to normal thresholds requires from two to seven days. Chinchillas were exposed to this same noise for two days at 65, 75, 85, or 95 dB SPL. The organ of Corti was immediately fixed and prepared as a flat, whole‐mount specimen in plastic suitable for both phase contrast and electron microscopy. Other ears exposed at 95 dB SPL were allowed to recover 7, 28, or 70 days before fixation. The number of cisternae in the peripheral membrane system of many outer hair cells was increased from three to six to as many as 30. Similar membranes appeared as whorls within the cells. The proportion of altered cells was greatest in upper second and lower third turns and increased with exposure levels. Recovery toward normal membrane configuration was evident at seven days and only minor variations remained after 28 and 70 days.