Many mouse mutants have an apparent deficiency in their responsiveness to sound¹. Most of these mutants have other abnormalities in addition to their hearing deficit, and the only two which have been subjected to a detailed anatomical and physiological study, shaker-1 and Ames waltzer, also have motor abnormalities^2,3. The existence of such motor abnormalities throws some doubt on the usefulness of these two mutants as possible models for hereditary deafness in man, which is most frequently uncomplicated⁴. Deol and Kocher have described the deafness mutation in which mice homozygous for the recessive deafness gene (dn/dn) were unresponsive to sound and had no significant behavioural abnormality⁵. Cochlear hair cells in deafness mice develop normally and then degenerate, and the adult animals are completely deaf⁶. We have now studied deafness mice in order to determine the nature of their inherited deafness. Our data indicate that stimulus-related cochlear potentials do not develop even though hair cells are present in the young animal. The endocochlear potential is present in the scala media, but behaves abnormally during anoxia.