NADPH is an essential cofactor for the function of the NOS enzyme. Unfortunately, NADPH interferes with the chemistry of the Griess reagents, which are the most commonly used reagents for nitrite detection. This kit uses Lactate Dehydrogenase (LDH) to oxidize the excess NADPH used in a NOS-catalyzed reaction, thereby making the assay particularly well suited to measurements of NOS activity in vitro. High throughput screening applications using recombinant NOS preparations can be quickly and accurately assayed with the LDH method kit.