Literature presents divergent hypotheses on the histogenesis of the subthalamic nucleus. The basal plate of the mesencephalon, the dorsal hypothalamus (or the longitudinal subthalamic zone) and an area closely related to the mammillary nucleus have all been proposed as the site of origin of the neurons of the subthalamic nucleus. However, based on the selective labelling of neurons of this nucleus by the [3H]thymidine ARG technique, our results clearly show that a germinative zone lying caudally along the dorsal aspect of the mammillary recess is responsible for the formation of the neurons of the subthalamic nucleus. From this site of origin, the neurons migrate radially and then tangentially and dorsally along the marginal layer of the ventral diencephalon. The present study gives a detailed account of the histogenesis of the subthalamic nucleus of the rat through successive stage of development. Our data also show that the same proliferative neuroepithelial matrix that gives rise to the subthalamic neurons is also responsible for the genesis of the neurons of the supramammillary and submammillothalamic nuclei. Some aspects of the cytological maturation of the neurons of the subthalamic nucleus are also given.