Summary Terminal patterns of afferent fibers to the interpositus and the dentate nuclei, and the dendritic pattern of these nuclear cells and intranuclear connections were investigated in rats and cats using the rapid Golgi and modified Golgi methods. (1) Three kinds of afferent fiber were distinguished by their fiber course, terminal pattern and fiber size: (A) thick axons terminating directly in the nuclei, which correspond to Purkinje cell axons, (B) medium-calibered, and (C) fine axons that pass through the nuclei giving off collaterals to nuclear cells. The latter two kinds of axon were identified as axons of extracerebellar origin. As in the fatigial nucleus, the interpositus and dentate nuclei are supplied by a number of collaterals derived from afferents to the cerebellar cortex and, furthermore, these afferents of extraneous origin and Purkinje cell axons converge onto nuclear cells, contributing to the formation of dense pericellular plexus. (2) Dendrites of large and medium-sized cells of the dentate nucleus exhibit a radiate type, extending in all directions for 400 μm on the average. Medium-sized cells of the anterior interpositus nucleus differ markedly from those of the posterior interpositus nucleus in their cell form and dendritic patterns. The former are provided with oval or spindle-shaped cell bodies and dendrites extending in a dorsoventral direction for 450 μm, while the latter are multipolar and extend their dendrites in all directions for 280 μm. Small cells of the interpositus and dentate nuclei are oval or spindle-shaped and extend their dendrites in all directions for 200 μm. (3) Axons of large and medium-sized cells of the nuclei contribute to the formation of the brachium conjunctivum, giving off fine and sparsely branched recurrent collaterals within the nuclei. These recurrent collaterals appear to terminate on small cells identified as short-axoned cells by their axonal patterns. This finding suggests that a certain feedback system would be established within the cerebellar nuclei between interneurons (small cells)_and projection neurons (large and medium-sized cells).