Griess reagent is used to detect nitrite photometrically. The reagent contains two chemicals, sulfanilic acid and N-(1-naphthalenediamine). Under acidic conditions sulfanilic acid is converted by nitrite to a diazonium salt, which readily couples with N-(1-naphthalenediamine) to form a highly colored azo dye that can be measured by absorbance at 548 nm. Under physiological conditionss, NO is unstable and is rapidly oxidized to a mixture of nitrite and nitrate. In order to measure NO levels indirectly from nitrite, nitrate must be reduced to nitrite enzymatically using nitrate reductase (not included) before performing the assay so that the total amount of nitrite can be measured. The kit includes 50 mL ready-to-use Griess reagent for 500 cuvette assays or 2500 microplate assays and 1 mL nitrite standard solution (1 mM).