Double-immunolabelling techniques were employed to investigate the distribution of smooth muscle alpha-actin (actin) in glial fibrillary acidic protein (GFAP)-positive cells in rat brain during early postnatal development and maturation and in glial primary culture derived from newborn rat brain. In addition the expression of desmin was studied in the glial primary cultures as a function of the differentiation of the cells. Comparison of the cultured astroglial cells at an early age with hepatic stellate cells derived from CCl4-induced cirrhotic rat liver, revealed features of the astrocytic cytoskeleton characteristic of myofibroblastic cells, i.e., strong expression of both myofibroblastic markers, actin and desmin. In astroglial cells with an initial morphology reminiscent of fibroblasts the non-filamentous perinuclear immunoreaction of GFAP increased with time at the expense of actin and, partially, desmin. GFAP filaments were spread throughout the cytoplasm of the cells which acquired stellate morphology. The alterations in the morphology of the cells and the distribution and intensity of staining for GFAP and actin during the differentiation of astrocytes in culture were similar to those observed in astrocytes during the maturation of the brain. In astrocytes from a newborn brain as well as in cirrhotic hepatic stellate cells, the area of immunoreaction of GFAP was reduced and confined mainly to the nuclear region. In contrast, the cells expressed actin throughout the cytoplasm. These findings may hint at a similar function of these regionally specialized perivascular myofibroblastic cells in a normal brain and diseased liver and at inverse organ-specific functions which the cells fulfill under non-pathological conditions in vivo.