Anatomical studies using light microscopy have shown that mice affected by thedeafnessorjerkergenes develop cochlear hair cells which at first appear to be normal. The hair cells then begin to degenerate, and by about three months of age most of the organ of Corti has degenerated. The object of the present study was to determine whether the cochleas of affected animals are functionally normal during the period when they appear structurally normal. Mice from the two strains aged between 12 and 50 days were anaesthetized and audiograms were determined by measuring thresholds for detecting the compound action potential recorded at the round window. No action potentials or cochlear microphonics could be recorded from affected animals at any age but littermates unaffected by the respective genes had normal thresholds. These results indicate that the apparent structural integrity of the cochleas of youngdeafnessandjerkermice is not accompanied by normal function. Mice affected by these genes appear to be completely deaf at all stages of cochlear development.