The morphology of geniculo-cortical relay neurons (GCR-neurons) of tree shrew, rat and cat differs intraspecifically and interspecifically from each other. Intraspecific differences in the cat have been explained functionally by the W-, X-, and Y-characteristics of cells (Wilson et al. 1976). Analogous morpho-functional correlations may also explain the morphological differences in tree shrew and rat. The interspecific differences in the morphology of GCR-neurons seem to be closely correlated to the levels of visual specialization in the three species in which the rat represents a basal developmental stage.