The subcortical afferents of the mediodorsal thalamic nucleus (MD) were studied using the method of retrograde horseradish peroxidase transport. Analysis of the distribution of labeled cells following injections into particular MD segments established specific and unspecific projections originating in various subcortical structures. The medial MD segment is related to such structures of the limbic system as: the anterior olfactory nucleus, olfactory tubercle, nucleus of diagonal band, arnygdala and septum, whereas the lateral MD segment receives afferentation from such structures involved in motor activity regulation as: the substantia nigra, entopeduncular nucleus, and cerebellar nuclei.Moreover, this segment is connected with the structure of the primary visual system, the lateral geniculate body. Nonspecific projections terminating in the entire MD nucleus originate in numerous structures at different CNS levels. The hypothalamus and reticular thalamic nucleus seem to have the strongest relationship with MD.