Astrocytes in primary cultures constitute an exceedingly useful preparation for studies of astroglial development and function. These cells, however, demonstrate a pronounced plasticity in their reactions to culturing conditions. Thus, species and spatiotemporal region of CNS chosen for source of cells, dissociation procedures used, cell density in culture, culture medium chosen, type and/or concentration of serum used (if any) and exposure to dibutyryl cyclic AMP (dBcAMP) may all markedly affect the epigenotype of the cells. This provides an experimental tool for studies of astroglial development and function, but is at the same time a reason for caution in the interpretation of data when such cultured cells are used as models of their in vivo counterparts. About 95% of the cells in cultures used by the authors are positive for astrocyte-specific markers (glial fibrillary acidic protein and glutamine synthetase), and the cells possess presumably astrocytic characteristics such as high potassium permeability. This latter characteristic may be drastically altered by minor changes in culturing conditions. It is likely that an overwhelming majority of the astrocytes in such cultures are protoplasmic in nature. Addition of dBcAMP to the culture medium results in a pronounced morphological and a more modest functional differentiation.