The topography of the cerebellar nucleo-cortical projection was investigated in the cat by experiments employing the horseradish peroxidase (HRP) technique or by combined HRP-autoradiographic methods. The results of the HRP studies extend previous findings showing that neurons in the deep nuclei project to the cerebellar cortex in an orderly way. Thus, it appears that the cortex of the vermis-proper receives projections from neurons located predominately in the fastigial nucleus. Intermediate and lateral zones of mid-vermal cerebellar cortex are projected on by neurons located in the interposed and dentate nuclei. Crus II receives input from neurons located predominately in the dentate nucleus, while the paramedian lobule is projected on by neurons located in a large postero-dorsal sector of the interposed nucleus and in a smaller medial strip of the dentate nucleus. Neurons in the ventral part of the dentate nucleus and the lateral part of the interposed nucleus send fibers to the paraflocculus. The nucleo-cortical pathway to the flocculus and nodulus arises largely from a population of neurons located in a ventral region stretching from the medial border of the dentate nucleus to the lateral border of the fastigial nucleus. The results of experiments using the combined HRP-autoradiographic method show that clusters of neurons in the deep cerebellar nuclei project back to the cerebellar cortical areas from which they receive input, establishing a fairly precise feedback loop between the cerebellar cortex and deep nuclei.