This study discusses the histogenesis of the structures of the extrapyramidal system. The first part based on [3H]thymidine autoradiography provides data on the time of origin of the various neuronal populations that characterize different structures of the extrapyramidal system. Such data are essential to any further study dealing with the localization of the different sites of origin of the neurons along the ependymal matrix and with their migration during the histogenetic sequences leading to their definitive pattern of adult distribution. The neurons of globus pallidus and of the entopeduncular nucleus are generated on days 12-15 and days 11-14, respectively. A peak of neurogenesis occurs on day 12 for the entopeduncular nucleus and on day 14 for the globus pallidus. In the pallidum, the first neurons to form on day 12 settle caudally, while neurons generated on day 15 settle in the rostral extremity. The genesis of the medium-sized neurons of the neostriatum extends from day 12 to at least postnatal day 2. A peak is obvious on day 15. Although the neurogenesis of these neurons follows a mild caudorostral gradient, a more careful examination reveals four different patterns of settlement according to the area involved and the period of gestation. At the level of the caudal neostriatum, the neurons display a clear mediolateral spatiotemporal gradient. More rostrally, the neurons generated on days 13, 14 and 15 show two patterns of settlement. On the one hand, many neurons settle rather densely along the external capsule on day 13, occupying more rostral levels on days 14 and 15. On the other hand, in the body of the neostriatum, clusters of isochronically generated neurons are obvious. Later, newly generated neurons display a rather homogeneous distribution in the structure. A parallel is drawn between these patterns of development and the patterns of distribution of afferent terminals or neurotransmitters. The large chromophilic neurons of the neostriatum appear exclusively during the early period. Two peaks of neurogenesis are apparent. The one on day 13 comprises neurons that settle caudally. It is contemporaneous to the neurogenesis of the adjacent basal nucleus. The second peak occurs on day 15 and corresponds to that of the medium-sized neurons.