There is increasing evidence that nitric oxide (NO), a free radical that can act both as a signaling molecule and a neurotoxin, is involved in the mechanisms of cerebral ischemia. Although early investigations yielded conflicting results, the introduction of more-selective pharmacological tools and the use of molecular approaches for deletion of genes encoding for NO synthase have provided a better understanding of the role of NO in the mechanisms of ischemic brain damage. The evidence reviewed in this article suggests that NO is protective or destructive depending on the stage of evolution of the ischemic process and on the cellular source of NO. Defining the role of NO in cerebral ischemia provides the rationale for new neuroprotective strategies based on modulation of NO production in the post-ischemic brain.