The mammalian forebrain, including the dorsal lateral geniculate nucleus (LGNd) and the visual cortex, continues both structural and functional development postnatally and is therefore a useful model for the study of developmental processes in the central nervous system (CNS). We report here the first description and comparison of the structural development of individual, functionally identified neurones in the mammalian forebrain. This comparison is made for the three main cell groups of the central visual pathways (W-, X- and Y-cells), in the neonate and the adult. In the adult, these three classes of neurones have different characteristic electrophysiological properties(1-9) and relay information in parallel about different features of a visual scene8,9 from the retina through the LGNd to the visual cortex. In addition, each of the three functional cell types has a characteristic structure in the adult10,11. By injection of the enzyme marker substance, horseradish peroxidase, into electrophysiologically identified neurones, the present study demonstrates that each of these functional classes of neurones also has a characteristic morphology in the neonate (in the LGNd of kitten 3-4 postnatal weeks of age). However, striking differences in the rates of maturation are seen. The W-cells are already mature at this age. The X-cells are the least developed. Surprisingly, some Y-cells are mature. Due to the susceptibility of Y-cells to an abnormal visual environment during development12-18, they had previously been thought to be slower to mature.