The histological features, development and pathogenesis of the inner ear degeneration in the deaf white cat has been studied in 57 kittens bred from 4 female and 2 male parent animals. The degeneration makes its appearance between the 4th and 6th day of post-natal life, and up to this time no histological abnormalities can be recognized in the inner ear. The degeneration is confined to the cochlea and saccule. In the cochlea a progressive degeneration of Corti's organ is seen and appears to be associated with volume oscillations of the endolymph. Atrophy of the stria vascularis also occurs. The saccule collapses but its fluid connexions with the endolymphatic duct remain patent. No significant changes are seen in other parts of the inner ear, including the saccus endolymphaticus and the vestibular system. Analysis of the genetic data shows that the white coat colour and inner ear degeneration are due to the effect of a single dominant gene, the penetrance for the white coat colour being complete and that for the inner ear degeneration, 80%. Points of interest are that in a small number of white animals inner ear degeneration is unilateral in some and absent altogether in others. It is thought that the inner ear degeneration is essentially a derangement of the water electrolyte regulating mechanism of the inner ear. In this the gene is thought to play a part by reducing the enzyme content of the developing organ in its critical maturation phase and is supplemented in this action by certain unfavourable environmental factors. Similar mechanisms are thought to be involved in the pathogenesis of the inner ear degeneration which is commonly present in cases of human congenital deafness, not only when this is genetically determined but also in a certain important group of individuals in whom genetic factors appear to be absent.