The site and time of origin, as well as the migratory movements and gradients of histogenesis displayed by cells destined to form the medial thalamic nuclei, have been studied using autoradiographic techniques. 53 embryos and newborn rabbits, whose mothers were injected intraperitoneally with a single dose of tritiated thymidine from the 15th through the 22nd day of gestation, were used. Cells destined to form the medial thalamic nuclei originate in the matrix layer surrounding the third ventricle. From the matrix, cells migrate to form the different nuclei in an orderly fashion according to three definite gradients of histogenesis. One gradient has an outside-inside sequence showing that the lateral sectors form earlier than the medial sectors. A second gradient advances ventrodorsally and the third one is a postero-anterior gradient which demonstrates that the structures located caudally are more advanced in their histogenesis than the frontal structures. It is worth stressing the orderly relationships that exist between the time of origin of the thalamic cells and their definitive positions within the thalamus, which proves that the histogenetic process does not depend on the morphological limits of the region. In addition, the structures that constitute the independent and dependent thalamus, that are analyzed in this study, develop synchronously according to their position within the thalamus, suggesting that early connections would not be responsible for the particular patterns of histogenesis present in the development of the thalamus.