Cells of astroglial lineage in the murine cerebrum undergo a succession of transformations during prenatal and early postnatal development. The bipolar radial cell, the earliest astroglial form to appear, provides a radially aligned, parallel array of fibers that serves as a guide to neuronal migration. The multipolar astrocyte is the representative of this lineage that persists in the adult cerebrum. The processes of the multipolar astrocytes form a complex reticulum, which is considered critical to the development, function, and maintenance of neural circuits. A monopolar radial cell appears to be transitional between the two. The shift from the radial glial fiber system to a diffuse glial network is achieved largely in the E17-P2 interval in the mouse. This phenomenon has been studied qualitatively and quantitatively by staining cerebral tissue with monoclonal antibody RC2, a specific and sensitive ligand for cells of astroglial lineage in the mouse. Elongation and branching of glial processes contribute to the glial transformation. Elongation of radial fibers occurs under the guidance of other radial glial fibers (fasciculated elongation) or independently of other fibers (nonfasciculated elongation). Fasciculated elongation results in an increase in the density of radial glial fibers that span the cortical layers. Nonfasciculated elongation appears to be associated with process branching. This is the initial event in transformation of the bipolar radial cells to monopolar radial or multipolar cells. Only nonfasciculated elongation is characteristic of processes of the monopolar radial cells and multipolar astrocytes. Branching of the processes of all three cell forms appears to occur both by bifurcation at the elongating tip and by sprouting from the fiber shaft. Elongating fibers are tipped by growth cones that are relatively simple in shape as compared to those observed at the tips of elongating axons. Growth cones at the tips of nonfasciculated fibers are more complex in form than those at the tips of radial fibers elongating in contact with other radial fibers.