The Cayman Chemical Nitrate/Nitrite Assay Kit provides an accurate and convenient method for measurement of total nitrate/nitrite concentration in a simple two-step process. The first step is the conversion of nitrate to nitrite utilizing nitrate reductase. The second step is the addition of the Griess Reagents which convert nitrite into a deep purple azo compound. Photometric measurement of the absorbance due to this azo chromophore accurately determines nitrite concentration. This kit can be used to measure nitrate and nitrite in plasma/serum, urine, tissue culture media, and tissue homogenates. However, it cannot be used to analyze nitrate and nitrite from an in vitro assay of nitric oxide synthase (NOS) in which excess NADPH has been added. For these assays a second method (LDH method) is utilized and is available from Cayman Chemical in a 96 well plate format (Cay-760871).