Astrogliosis in response to injury usually represents up-regulation of glial fibrillary acidic protein (GFAP), hypertrophy, and proliferation. Following pathological events in brain tissue, purine nucleotides and nucleosides are released into the extracellular space. The (patho)physiological importance and molecular mechanisms of the purinoceptor-mediated effects are nearly unknown. In the present study, the involvement of extracellular ATP in astrogliotic processes via stimulation of P2 receptors was investigated. The structural analogue, 2-methylthio ATP (2-MeSATP) and its antagonists reactive blue 2 and pyridoxal-phosphate-6-azophenyl-2,4-disulphonic acid (PPADS) were microinfused in situ into the rat nucleus accumbens. The reaction of astrocytes in the nucleus accumbens was investigated by GFAP-and 5-bromo-2Јdeoxyuridine (BrdU)-immunocytochemistry. Tissue injury due to the microinjection procedure caused astrogliosis, which was increased further by 2-MeSATP. Up-regulation of GFAP-immunoreactivity, hypertrophy of astrocytes, and an increase in the number of GFAP-positive and of GFAP-/BrdU-double labeled cells were observed. Reactive blue 2 and PPADS decreased the consequences of tissue injury on astrocytic proliferation when given alone. In addition, both antagonists counteracted the 2-MeSATP-induced astrogliosis, supporting the hypothesis that purine nucleotides are involved in these processes via stimulation of P2 receptors in vivo.