The dynamic responses of the hearing organ to acoustic overstimulation were investigated using the guinea pig isolated temporal bone preparation. The organ was loaded with the fluorescent Ca²⁺ indicator Fluo-3, and the cochlear electric responses to low-level tones were recorded through a microelectrode in the scala media. After overstimulation, the amplitude of the cochlear potentials decreased significantly. In some cases, rapid recovery was seen with the potentials returning to their initial amplitude. In 12 of 14 cases in which overstimulation gave a decrease in the cochlear responses, significant elevations of the cytoplasmic [Ca²⁺] in the outer hair cells were seen. [Ca²⁺] increases appeared immediately after terminating the overstimulation, with partial recovery taking place in the ensuing 30 min in some preparations. Such [Ca²⁺] changes were not seen in preparations that were stimulated at levels that did not cause an amplitude change in the cochlear potentials. The overstimulation also gave rise to a contraction, evident as a decrease of the width of the organ of Corti. The average contraction in 10 preparations was 9 μm (SE 2 μm). Partial or complete recovery was seen within 30–45 min after the overstimulation. The [Ca²⁺] changes and the contraction are likely to produce major functional alterations and consequently are suggested to be a factor contributing strongly to the loss of function seen after exposure to loud sounds.