The time of origin of the amygdaloid complex was studied in mice injected once with tritiated thymidine during gestation or early neonatal development. Neurons for this nucleus and related nuclei arose during embryonic days 11-15. No mediolateral or dorsoventral gradients, but a distinct rostrocaudal gradient of proliferation was evident in comparisons of both rostral vs caudal nuclei and of anterior vs posterior regions of individual nuclei, such as the cortical nucleus. Since the pattern of development of this controversial nucleus was similar to that of nuclei in both centromedial and basolateral groups, time of origin data could not aid its assignment to one of these subdivisions.