Glutamine synthetase (GS), the enzyme that catalyses glutamine synthesis from glutamate and ammonia, plays a central role in the detoxification of brain ammonia. In the central nervous system (CNS), GS also subserves additional important functions such as regulating glutamate, GABA and amino acid metabolism. Oligodendrocytes (OL) form the myelin sheath in the central nervous system (CNS) and are essential for efficient propagation of nerve impulses. In culture, OL arise from bipotential O-2A progenitor cells. These O-2A cells give rise to type-2 astrocytes in the presence of serum. GS is expressed in mature glial cells in vivo and in vitro, but it is unknown whether GS is present in glial progenitors. In addition, a comparison of the GS expression level among the various types of glial cells has never been done in vitro. The current study investigates in vitro GS expression levels in O-2A progenitors, astrocytes and OL. We demonstrate that the GS gene is expressed in O-2A progenitors and is expressed at different levels in each cultured glial cell type. GS also is stimulated during OL developmental maturation. Thus, the GS gene is expressed in O-2A cells and is regulated in a developmental and macroglial cell type-specific manner.Glutamine synthetase (GS), the enzyme that catalyses glutamine synthesis from glutamate and ammonia, plays a central role in the detoxification of brain ammonia. In the central nervous system (CNS), GS also subserves additional important functions such as regulating glutamate, GABA and amino acid metabolism. Oligodendrocytes (OL) form the myelin sheath in the central nervous system (CNS) and are essential for efficient propagation of nerve impulses. In culture, OL arise from bipotential O-2A progenitor cells. These O-2A cells give rise to type-2 astrocytes in the presence of serum. GS is expressed in mature glial cells in vivo and in vitro, but it is unknown whether GS is present in glial progenitors. In addition, a comparison of the GS expression level among the various types of glial cells has never been done in vitro. The current study investigates in vitro GS expression levels in O-2A progenitors, astrocytes and OL. We demonstrate that the GS gene is expressed in O-2A progenitors and is expressed at different levels in each cultured glial cell type. GS also is stimulated during OL developmental maturation. Thus, the GS gene is expressed in O-2A cells and is regulated in a developmental and macroglial cell type-specific manner.