Experiments were performed to determine if a nucleocortical system, a projection from the cerebellar nuclei to the cerebellar cortex, was present in primates. Both electrophysiological and neuroanatomical techniques were employed to investigate this question. It was shown that neurons within the dentate and interposed nuclei were antidromically activated by stimuli applied to the cerebellar cortex. In addition, cells in these nuclei were retrogradely labelled following injections of small amounts of horseradish peroxidase in the cerebellar cortex. The injection of tritiated leucine in the deep nuclei resulted in the labelling of fibers projecting from these structures to the cerebellar cortex which appeared to terminate within the granular layer. Additional electrophysiological studies showed that neurons projecting to the cerebellar cortex could also be antidromically activated from the ventrolateral thalamic nucleus, indicating that the nucleocortical projection in the primate arises at least in part as collaterals from neurons in the deep cerebellar nuclei which also project to extracerebellar structures, as was shown in the cat.