As the taxonomic status of the Tupaioidea within the Order Primates is still unsettled, an attempt is being made to throw some light on this problem through investigations on the nuclear configuration and fibre connections of the diencephalon of the tree-shrew. Several tree-shrew species, particularly Tupaia glis and Urogale everetti have been involved in this qualitative study, and comparisons of diencephalic structures among the tree-shrews species, and with the Insectivores and Prosimii have been made. Thalamic and hypothalamic nuclei which show more advanced development than others, are nuclei anteroventralis, mediodorsalis, centrum medianum, lateralis posterior, pulvinaris, geniculati lateralis and medialis, subthalamicus and mamillaris medialis. The nucleus mediodorsalis becomes much enlarged and more clearly differentiated into two or three regions. The ventrolateral thalamic nuclear group, though comparably small, is clearly definable into anterior, lateral and posterior parts. The pulvinar makes its first definitive appearance as a separate entity. The nuclei pretectalis and thalamicus posterior are larger and more prominent than those in the Prosimii. The dorsal part of the lateral geniculate body, termed nucleus geniculatus lateralis, show a clear stratification of its cells into six layers that correspond well to the primate pattern. The nucleus geniculatus medialis is well developed and differentiated into magnocellular and parvocellular parts. The epithalamus, subthalamus and hypothalamus are not much different from those of other mammalian species. This study has shown that the thalamus and metathalamus show more advanced than primitive phylogenetic features than are found in the Insectivora. These may compare favorably with most of the homologues in the diencephalon of the Lemuroidea.