AbstractTransforming growth factor-α-like immunoreactivity (TGF-α-ir) was visualized in the adult rat forebrain using three antisera directed against carboxyterminal sequences in the TGF-αprecursor. Using immunoperoxidase and immunofluorescence techniques with all three antisera, TGF-λ-ir was found to be present in a subpopulation of astrocytes in the forebrain. Striatal and pallidal regions of the basal ganglia were studied in detail. In the striatum, there was an uneven distribution of astrocytes containing TGF-α-ir, with the greatest number in the dorsal medial third of the caudate-putamen and the overlying corpus callosum/external capsule. In addition, the region of the caudate-putamen bordering the globus pallidus contained numerous clusters of TGF-α-ir astrocytes. The globus pallidus itself contained numerous and more evenly distributed TGF-α-ir astrocytes. Other pallidal structures-including the ventral pallidum, entopeduncular nucleus, and substantia nigra pars reticulata–contained moderate numbers of TGF-α-ir astrocytes. These results suggest that TGF-αprecursor is present and, perhaps, synthesized in astrocytes. A related growth factor, epidermal growth factor (EGF), has also been reported to be present in pallidal regions of rat brain. Therefore, the TGF-α/EGF family of trophic factors may play an important role in the function of the central nervous system.