There is an abundance of evidence that nitric oxide plays a critical role in regulating several components of gastrointestinal mucosal defense. Suppression of nitric oxide synthesis increases susceptibility to injury, while administration of nitric oxide donors increases resistance to injury. On the other hand, nitric oxide has been implicated as a mediator of tissue injury in the gastrointestinal tract during inflammatory reactions. In these cases, the nitric oxide is generally believed to be derived from an inducible isoform of nitric oxide synthase. In this review, we provide an overview of the evidence for and against these dual roles of nitric oxide in modulating gastrointestinal mucosal defense and injury. We also highlight the potential therapeutic benefits that may be realized through modulation of tissue nitric oxide levels.