Many mouse mutants have an apparent deficiency in their responsiveness to sound. 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. 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 uncomplicated. 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 abnormality. Cochlear hair cells in deafness mice develop normally and then degenerate, and the adult animals are completely deaf. 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.