Recent human studies suggest that abnormal development of oligodendrocytes (OLs) is an important component in the pathophysiology of schizophrenia. However, less information is available regarding effects of antipsychotics on OLs' development. In the present study, young adult C57BL/6 mice were given haloperidol (HAL; 2mg/kg/day) in their drinking water for three or six weeks. At the conclusion of the drug treatment, mice were sacrificed and the numbers of NG2- and Olig2-expressing cells in the brain regions of the corpus callosum, hippocampus and cerebral cortex were quantified. NG2 is a specific marker for oligodendroglia precursor cells (OPCs); Olig2 marks glial progenitors. HAL treatment for three weeks increased the number of NG2-expressing cells in the corpus callosum; HAL treatment for three and six weeks increased the numbers of Olig2-expressing cells in all three brain regions and increased the levels of Olig2 expression in the same brain regions. These results suggest that HAL treatment activates adult OPCs, which divide infrequently under normal conditions but respond to a variety of insulting factors by proliferation and differentiation. However, our further observations showed no changes in the number of mature OLs and the amount of myelin basic protein in HAL-treated mice, suggesting the drug treatment has no effect on the maturation of OLs. In addition, HAL treatment did not increase the numbers of GFAP- and CD68-expressing cells, suggesting that no gliosis and inflammatory responses occurred while the drug activated the quiescent OPCs in adult brain. These results suggest that HAL treatment may target the development of OLs.