This study provides basic data on the time of origin as well as on the spatio-temporal setting pattern of the neurons of the red nucleus, the inferior olivary complex and the cerebellar nuclei during ontogenesis of the rat brain. The first neurons of the olive form on day 12. In the adult they set in the rostral part of the dorsal accessory olive and in the minor subgroups associated with the medial accessory olive. On day 13, neurogenesis is intense in both divisions of the red nucleus, in all areas of the inferior olivary complex, in the medial and interpositus cerebellar nuclei and in the small-celled part of the lateral cerebellar nucleus. On day 14, neurogenesis is completed in the magnocellular division of the red nucleus and in the dorsal accessory olive but the neurons of the principal and medial accessory olives, of the rostral and caudal poles of the medial cerebellar nuclei, of the interpositus and lateral cerebellar nuclei as well as those of the parvocellular division of the red nucleus are still formed. Our study suggests that in the rat brain the neurons of the nuclei involved in the rubro (parvo)-olivo-cerebello-rubral and rubro (magno)-reticulo-cerebello-rubral loops develop on days 13 and 14 of gestation. Maturation, cytoarchitecture and phylogeny are also discussed.