In cerebral ischemic insults, activated inflammatory cells such as microglia and macrophages may be implicated in the pattern and degree of ischemic injury by producing various bioactive mediators. In the present study, we provide the evidence that activated microglia/macrophages accelerate cerebral ischemic injury by overexpression of inducible nitric oxide synthase (iNOS). To activate microglia/macrophages, a potent inflammation inducer lipopolysaccharide (LPS, 5 μg/5 μl) was microinjected into rat corpus callosum. Isolectin B4-positive microglia/macrophages were abundantly observed in ipsilateral hemisphere at 1 day after LPS injection. RT-PCR showed that LPS injection induced iNOS mRNA expression mostly in microglia/macrophages, peaking in intensity at 15 h after LPS injection. While ischemic injury was little evoked in control rats by 2-h middle cerebral artery occlusion (MCAO) followed by 3-h reperfusion, it was markedly increased in rats pre-injected with LPS 1 day before MCAO. However, no significant difference between control and LPS-pretreated groups was observed after 24-h reperfusion. The increased ischemic injury in LPS-treated rats was well correlated with iNOS level expressed over 3 orders of magnitude than in LPS-untreated rats. Immunohistochemical studies showed that iNOS- and nitrotyrosine (a peroxynitrite marker)-positive cells were prominent throughout the infarct area. NOS inhibitors aminoguanidine or NG-nitro-L-arginine, simultaneously injected with LPS, reduced the iNOS immunoreactivity and infarct volume, especially in penumbra regions. Total glutathione levels in ischemic regions were decreased more in LPS pre-injected rats than in control ones. Further defining the role of NO in cerebral ischemic insults would provide the rationale for new therapeutic strategies based on modulation of microglial and macrophageal NO production in the brain. © 2005 Wiley-Liss, Inc.