Extracellular nucleotides, including ATP, have been demonstrated to transmit important physiological signals in the brain through either G-protein-coupled P2Y receptors or P2X receptors, which are ligand-gated ion channels. In this study, we performed a detailed analysis of the expression of the Gi/o-coupled receptor P2Y12 in the brain. Northern blot analysis demonstrated that P2Y12 is expressed predominantly in the brain, and to a lesser extent in the spleen. The cellular localization of P2Y12 was investigated by in situ hybridization, and P2Y12 mRNA was detected in small cells distributed throughout the brain, including the hippocampus. Expression of P2Y12 was also observed in naive and axotomized facial nuclei, and the number of P2Y12-expressing cells increased following facial nerve axotomy. Selective expression of P2Y 12 mRNA in microglia was confirmed by double-label in situ hybridization and immunohistochemistry with antibodies against NeuN and Iba1 as an immunohistochemical marker for neurons and microglia, respectively. Hardly any P2Y12 mRNA was detected in macrophages obtained from the spleen and abdominal cavity, which share many surface molecules with microglia.