This study assessed the contribution of the apical hair cells to hearing. Guinea pigs, chinchillas and monkeys were behaviorally trained using positive reinforcement to respond to pure-tone stimuli. When a stable audiogram had been determined, each subject received one of three experimental treatments: ototoxic drug administration, low-frequency noise exposure, or the application of a cryoprobe to the bony wall of the cochlear apex. After post-treatment audiograms stabilized, subjects were euthanized and the percentage of hair cells remaining was assessed by light microscopy. Results indicate that a redundancy of encoding mechanisms exist in the mammalian cochlea for low-frequency stimuli. They also suggest that a very small percentage of apical hair cells are sufficient for some low-frequency hearing. Finally, data from this and other studies suggest that the low-frequency threshold shift caused by the loss of a certain percentage of apical hair cells is less pronounced than the high-frequency threshold shift caused by the loss of a comparable percentage of basal hair cells. These data agree with anatomical and electrophysiological evidence that functional as well as anatomical differences may exist between the apex and base of the cochlea.