It has been suggested that pre-natal and or peri-natal hypoxia can cause permanent hearing loss as a result of interference with the generation of the endocochlear potential, especially if this interference occurs during the critical period of endocochlear potential development. This hypothesis was tested in 4 series of experiments: in neonatal rats exposed to pure nitrogen for several minutes; in neonatal and adult cats respirated with hypoxic gas mixture for an hour; in neonatal rats and cats breathing hypoxic gas mixture for several hours; and in neonatal rats in hypobaric chamber for several days. In each experiment, the auditory nerve-brainstem evoked response threshold was elevated during hypoxia in those animals in which it could be measured but was normal when determined several hours to days later. This is evidence for a remarkable ability of the auditory system to recover from periods of hypoxia even during the critical period of endocochlear potential development.