The post‐neurogenic period in the mammalian neocortex is characterized by the growth of astrocyte and oligodendrocyte populations and their incorporation into the network of the developing central nervous system (CNS). Many of these glial cells originate as progenitors in the subventricular zone (SVZ) and then migrate into white and gray matter before differentiating. What determines the specific cellular fate of progenitors in vivo is not known, however. In examining the early stages of gliogenesis from progenitors in the SVZ, we noted that interactions with cortical blood vessels took place at what appeared to be an early stage of glial differentiation. We have examined in more detail the interactions of progenitors with blood vessels in the early postnatal rat neocortex after labeling progenitors in vivo with a LacZ‐encoding retrovirus. These early interactions are accompanied by an increase in intermediate filament expression, consistent with astrocytic differentiation. Because astrocytes interact with blood vessels and pia, we suggest that such contact represents an early stage in astrocytic differentiation. Furthermore, since angiogenesis and astrogenesis occur over a similar period, the growth of blood vessels may even play a role in the selection of astrocytic fate by a progenitor. As vessel growth slows, fewer progenitors may be directed toward an astrocyte fate, allowing more to differentiate into oligodendrocytes, perhaps explaining the shift from astrocyte genesis to oligodendrocyte genesis during early postnatal cortical development. J. Comp. Neurol. 387:537–546, 1997. © 1997 Wiley‐Liss, Inc.