There are four potentially granular or frankly granular cells within the connective tissue compartment of the mammalian central nervous system, whether this is part of the surface leptomeninges or the leptomeningeal sleeves around parenchymal blood vessels larger than capillaries. These are: (1) Cells that behave like macrophages, part of the mononuclear phagocyte system of the body; they are granular to varying degrees (containing lysosomes). (2) Brown-pigmented granular cells which are mainly located on the surface but are also seen for varying distances along blood vessels as they pass inside the CNS of pigmented animals. (3) Mast cells (MCs) which are granular and located especially prominently in surface leptomeninges of young mammals, and, in adults, are restricted to special parts of the CNS. (4) Granular cells, referred to by me as neurolipomastocytoid cells (NLMs), are numerous, ubiquitously distributed, and seem to have morphological features in common with those of both MCs and macrophages. The exact identiy of these NLMs still needs to established. One approach was to study the development of all three non – pigmented cells in the immature brain of the albino rat, especially at the ultrastructural level. This communication represents the findings regarding the MCs. The MCs appear to arise from a small mononuclear cell and to go through maturation stages identical to those described by others for MCs outside the CNS. The greatly flattened adjacent leptomeningeal cells are an easily identifiable entity especially’due to their peculiar glycogen content in the young.