The existence of divergent axon collaterals of neurons in the deep cerebellar nuclei has been investigated in rat by means of the fluorescent retrograde double labeling technique. The results have led to the following conclusions. A. Many of the neurons in the lateral, the interpositus as well as the caudal half of the medial nucleus project to the diencephalon. Some of these neurons distribute divergent axon collaterals to the superior colliculus, but few neurons project only to the latter structure. B. Some of the deep cerebellar neurons located laterally, i.e. in the dorsomedial part of the lateral nucleus, as well as some others located medially, i.e. in the medial part of the interpositus nucleus and the adjoining part of the medial nucleus, distribute divergent axon collaterals to the diencephalon and the spinal cord. C. Deep cerebellar neurons located laterally: in the cell group of the dorsolateral hump (Dlh) and in the adjoining lateral part of the interpositus nucleus, as well as some other located medially, i.e. in the dorsolateral part of the median nucleus (Mdlp), distribute divergent axon collaterals to the diencephalon and to the medulla oblongata, probably primarily its medial reticular formation. However, only few of the neurons, which distribute descending collaterals to the spinal cord or the medulla oblongata, distribute ascending collaterals to the superior colliculus. D. After injections in the medulla oblongata a population of small sized single labeled neurons was encountered especially in the lateral and interpositus nuclei. On the basis of other findings in rat they were assumed to represent cerebello-olivary neurons.