Combined neuroanatomical and electrophysiological experiments were performed to test the hypothesis that axon collaterals of neurons in the cerebellar nuclei project to the cerebellar cortex in cats. The anatomical studies demonstrated that (a) following the injection of tritiated leucine into the deep cerebellar nuclei, labeled fibers could be traced into the granular layer of the cerebellar cortex, and (b) following the injection of horseradish peroxidase into the cerebellar cortex, retrogradely labeled horseradish peroxidase-positive neurons were identified in the deep nuclei. The electrophysiological experiments confirmed the anatomical findings. Neurons in the dentate and interposed nuclei, identified by their antidromic activation from the brachium conjunctivum, could also be activated antidromically from the cerebellar surface. Collision experiments demonstrated that projections from the deep cerebellar nuclei to the cerebellar cortex are in part collaterals of efferent neurons projecting through the brachium conjunctivum. Care was taken to ensure that all recordings were obtained from the region of cell somata in order to minimize the likelihood of recording from neuronal elements passing through the cerebellar nuclei. These combined neuroanatomical and electrophysiological studies provide strong evidence supporting the existence of a collateral system from cerebellar output neurons to the cerebellar cortex. The existence of this collateral system emphasizes that the cerebellar cortex and cerebellar nuclei may comprise a functional unit in which these collaterals may serve as a substrate for feedback control of the cerebellar cortex by the cerebellar output.