The production of nitric oxide by the calcium-independent inducible nitric oxide synthase (iNOS) in glial cells has been implicated in the neuropathogenesis of various diseases. It is well known that in response to lipopolysaccharide (LPS) and cytokines, such as IFN-γ, glial cells are induced to synthesize large amount of nitric oxide (NO) (Bolaños et al., 1996; Nicoletti et al., 1998). The signaling transduction pathways for iNOS transcription in astroglial cells have however not yet been established. Because IFN-γ receptor chains are associated with Janus tyrosine kinases (JAK1 and JAK2) (Darnell et al., 1994), we analyzed the involvement of the JAK/STAT signal transduction pathway in iNOS expression. Our study shows increased JAK2 and STAT1α/β tyrosine phosphorylation in primary astroglial cell culture after treatment with IFN-γ and LPS. A temporal correlation was observed between JAK2 and STAT1α/β tyrosine phosphorylation, the appearance of interferon-regulatory factor-1 (IRF-1) mRNA and the iNOS expression. Inhibition experiments showed that JAK2 and STAT1α/β tyrosine phosphorylation were necessary for IFNγ-mediated iNOS induction in astroglial cells. We conclude that JAK2 and STAT1α/β tyrosine phosphorylation is an early event involved in the expression of iNOS in astroglial cells. J. Neurosci. Res. 65:417–424, 2001. © 2001 Wiley-Liss, Inc.