Griess reagent is used to detect nitrite photometrically. The reagent contains two chemicals, sulfanilic acid andN-(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 detected at 548 nm (Figure 5.1). 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 ready-to-use Griess reagent, nitrite standard solution, and a detailed protocol.