Nitric oxide (sometimes called nitrogen monoxide) has the chemical formula NO. It is a colorless gas at standard temperature and pressure and it is slightly toxic. Nitric oxide is generally prepared by reacting copper and nitric acid or industrially by oxidation of ammonia over a catalyst (this is the first step in the manufacture of nitric oxide by the Ostwald process). At high pressure decomposition occurs to give other oxides of nitrogen, chiefly N₂O and NO₂. This latter oxide can also be produced by oxidation with atmospheric oxygen. Nitric oxide re acts with halogens to produce a compound with the chemical formula of halogenNO. These are the nitrosyl halides that act as strong oxidizing agents. Loss of an electron will give NO⁺, the nitrosonium ion which is also a strong oxidizing agent.

Nitric oxide is an important hormone in the human body. It causes the muscles surrounding blood vessels to relax, which allows an increased passage of blood. It can also help the immune system kill invading parasites.

Nitric oxide is also present in the atmosphere. Atmospheric nitric oxide is either naturally occurring (produced by electrical discharges) or is present as a pollutant from the combustion of organic matter and also from the internal combustion engine. In non-polluted air nitric oxide is found at levels of 0.01 ppm, but in smog it can be present in concentrations as high as 0.2 ppm.