In thymidine radiograms and plastic-embedded sections, the migration of cerebellar deep neurons was traced from their germinal source to their final settling sites. The route proved to be roundabout and three developmental events could be distinguished during the process. First, between days E14 and E16, transversely oriented cells of the nuclear transitory zone move in an arc from the ventrolateral neuroepithelium of the lateral cerebellar primordium in a medial direction. Second, between days E16 and E18, the cells of the rostral component of the nuclear transitory zone assume a longitudinal orientation. We postulated that this is the period of axonogenesis, the longitudinally oriented cells issuing efferents that join the superior cerebellar peduncle ipsilaterally and the transversely oriented cells (representing the neurons of the caudal fastigial nucleus) sending decussating fibers to the uncinate fasciculus (the hook bundle of Russell). Third, between days E18 and E21, the earlier-produced superficial cells of the nuclear transitory zone and the later-produced deep cells of the cortical transitory zone (the young Purkinje cells) exchange positions. The descent of the deep neurons is in the direction of the fibers of the inferior cerebellar peduncle, which becomes distributed throughout the cerebellum on day E17. The ascent of the Purkinje cells is in the direction of the external germinal layer, which begins to spread from caudal to rostral on day E17. The three deep nuclei, the lateral (dentate), interpositus, and medial (fastigial), can be distinguished before their descent into the depth of the cerebellum, and by day E22 a small-celled and a large-celled subdivision is identifiable in each nucleus.