Nitric oxide, or NO^., is a gas under atmospheric conditions. It is noxious because of its free-radical structure. It is biosynthesized from the amino acid L-arginine. The responsible enzymes are widely distributed in the human body. It has been shown that this simple molecule, NO^., plays important roles in mammalian physiology. It is one of the factors regulating vascular tone and blood pressure, inhibition of platelet aggregation, neurotransmission in the peripheral and central nervous systems and macrophage function. Evidence for the pathophysiological significance of NO^. is now accumulating.