The tiny diatomic gas nitric oxide is synthezised in biological systems by complex enzymes known as nitric oxide synthases. Recently, a novel pathway for NO production in humans was discovered that involves chemical reduction of inorganic nitrite, a reaction that takes place predominantly during acidic and reducing conditions. Nonenzymatic NO production has been demonstrated, e.g., in the stomach, on the surface of the skin, in the ischemic heart, and in infected nitrite-containing urine. NO produced by this mechanism is likely to play a role in similar biological events as when produced from L-arginine by NO synthases.