One of the most rapidly advancing areas of clinical application is the inhalation of low concentrations of the NO gas. Interestingly, nitric oxide at low concentrations is quite stable and minimally reactive. Its reactivity and toxicity is a second order reaction dependent upon the concentration of nitric oxide and its interaction with other free radicals and reactive oxygen species such as superoxide. At higher concentrations NO can interact with many transition metals, heme-containing proteins, thiol groups and can oxidize functionalities on polynucleotides (RNA and DNA) and proteins and can form strand breaks in polynucleotides. There is a rapid and almost diffusion-limited interaction of nitric oxide with superoxide to form the very reactive peroxynitrite.