Sprague-Dawley albino rats ranging in age from neonate to 60 days postnatal (dpn)were subjected to cortical extirpations encompassing the SmI somato-sensory projection fields of neurons in the ventrobasal (VB) complex. Eectron microscopy of this region reveals degenerative changes in VB neurons, the rate and severity of which is inversely proportional to the age of the animal (Matthews et al., 1977). Numerous, distinctive non-neuronal elements, similar to those infiltrating the perivascular space of some vessels in the area, rapidly accumulate within the zone of degeneration in animals lesioned between 0 and 9 dpn. These display dense, heterochromatin nuclei, concentrations of of free ribosomes and rosettes, and pleomorphic dense bodies which become more evident as further reactive transformations accompany the phagocytic incorporation of degenerating neuronal remnants. Other non-neuronal elements exhibit a euchromatin nucleus, bundles of microtubules, and fewer free ribosomes. Such cells are also capable of phagocytosis and production of dense bodies. Both variants are comparable in appearance to the "M" cells of previous reports (Matthews and Kruger, 1973b). Cortical lesions of older animals result in the appearance of "M" cells in VB; however, the population densities observed in the immature VB are not achieved. Conversely, astrocytic hypertrophy, associated with the increased incidence of degenerating boutons in the more mature animal, represents a prominent response to injury which does not occur to a significant extent in younger animals. Morphological criteria for determining the nature of some "M" cells are given for a discussion of their presumptive derivation from various mesodermal progenitors and a brief consideration of other hypothesized origins.