The morphology and histology of the cerebellar nuclei in the cat have been studied in serial section stained for cells and fibres and cut in the three conventional planes. The view first advocated by Weidenreich, viz. that the deep cerebellar nuclei of the cat comprise four divisions, here named nucleus medialis, nucleus interpositus posterior, nucleus interpositus anterior and nucleus lateralis, is confirmed. In the nucleus medialis which is the homologue of the nucleus fastigii of man, we have tentatively distinguished a sub-nucleus medialis parvocellularis which occupies a ventral position in the middle third of the medial nucleus. The nucleus interpositus posterior is closely associated with the nucleus medialis and presumably homologous with the nucleus globosus of man. The nucleus interpositus anterior is continuous with the nucleus lateralis, and presumably homologous with the nucleus emboliformis of man. The nucleus lateralis is composed chiefly of middle sized and large cells, with a definite small-celled ventral cap, subnucleus lateralis parvocellularis. The lateral nucleus presumably is homologous with the nucleus dentatus of man.