Nitrotyrosine (NO) has been identified as a marker of inflammation and NO production. Nitrotyrosine is formed in presence of the active metabolite NO. Nitrosylation of the amino acid tyrosine occurs both for free tyrosine and protein bound tyrosine. Various pathways including the formation of peroxinitrite lead to nitrotyrosine production. Since nitrotyrosine is a stable end product of peroxynitrite oxidation, assessment of its plasma concentration may be useful as a marker of NO-dependent damage in vivo. Since nitrogen oxide species (NOX) is only an indicator for enhanced NO production, protein associated nitrotyrosine might be a more suitable marker for damage induced by reactive nitrogen intermediates derived from NO. Furthermore, most proteins have a longer half life in the circulation than NOX levels. The presence of nitrotyrosine has been detected in various inflammatory processes including atherosclerotic plaques, celiac disease, rheumatoid arthritis, chronic renal failure and septic shock. In normal serum and plasma nitrotyrosine levels are low or sometimes even undetectable.