Nitric oxide (NO) is a gas with diverse biological activities produced from arginine by NO synthases. It is capable of interacting with a number of molecules, most notably superoxide, forming peroxynitrite, which, in turn, can mediate bactericidal or cytotoxic reactions. Nitric oxide also mediates smooth muscle relaxation, neurotransmission, and modulation of inflammation in a number of organ systems and pathophysiologic conditions. Modulation of NO by administration of inhaled NO for respiratory distress syndromes and infusion of NO synthase inhibitors in bacterial sepsis are ongoing. Levels of exhaled NO are being evaluated for their utility in assessing inflammation in respiratory disorders such as asthma.