Previous studies have provided evidence that the morphological organization of immature astrocytes is influenced by the inhibitory neuronal transmitter gamma amino-butyric acid (GABA). The present study was designed to determine whether the occurrence of differential organization of mature astrocytes throughout various regions of the adult brain is related to differential GABAergic signaling. For this we first used Western blotting and high-performance liquid chromatography to quantify the levels of the astrocytic protein glial fibrillary acidic protein (GFAP) and GABA, respectively, within the same tissue punches taken from different forebrain regions of the adult rat, as well as immunocytochemistry for GFAP, GABA, or glutamate decarboxylase to visualize the morphological organization of astrocytes and of GABAergic axons in these regions. These data indicate that GFAP and GABA contents are correlated throughout the different forebrain regions analyzed, and that the regions containing the highest densities in GABAergic terminals are those that contain astrocytes exhibiting the highest degree of stellation. Secondly, we chronically increased GABAergic signaling in vivo by the systemic administration of an inhibitor of GABA transaminase or by the intracerebroventricular infusion of muscimol, a potent agonist of GABAA receptors. Our data show that in both cases, the GFAP content of the different forebrain regions is significantly augmented, in close association with a marked increase in the number of astrocytic processes and with their degree of branching. Taken together, these data strongly suggest that GABAergic signaling mediates the morphological organization of astrocytes and their expression of GFAP in the adult brain. GLIA 41:137–151, 2003. © 2003 Wiley-Liss, Inc.