This nitric oxide assay kit is for quantitative determination of nitrite (NO2-) and nitrate (NO3-) in biological fluids. The kit uses the enzyme nitrate reductase to convert nitrate to nitrite. Nitrite is then detected as a colored azo dye product of the Griess reaction that absorbs visible light at 540nm. The interaction of nitric oxide (NO) in a system is measured by the determination of both nitrate and nitrite concentrations in the sample. The relative levels of nitrite and nitrate can vary substantially, therefore the most accurate determination of total NO production requires measurement of both nitrate and nitrite.