There is no disease of animals directly comparable with human cerebellar ataxia, although conditions variously designated as agenesis, hypoplasia, and atrophy are described. Dow and Moruzzi6have reviewed this topic. A condition in the cat called olivopontocerebellar atrophy was reported by Schut17as one of a number of recorded instances of hypoplasia in this species. Earlier, that topic had been reviewed by Brouwer,3who noted the association of cerebellar hypoplasia with atrophy of the inferior olives and of the pontine nuclei. Brouwer and Biermond4designated this condition, too, as olivopontocerebellar atrophy. In relation to the present study, it is important that Brouwer3described the deep cerebellar nuclei as showing a usual cell content. It is also relevant that in the ataxias of man and of cat the tissue studied represents an end stage of the pathological process. In the study reported herein, clinical manifestations become