NO is a multifunctional cell signaling molecule produced through the action of NOS. Cayman's Nitric Oxide Fluorometric Cell-Based Assay Kit detects NO in living cells by using fluorescein amine methyl ester (FA-OMe). When the non-fluorescent FA-OMe senses free NO in living cells it forms an intensely fluorescent deamination product, dA-FA-OMe whose fluorescence is directly proportional to the NO being produced by cells. dA-FA-OMe fluorescence can also be used to indirectly examine the NOS activity responsible for NO production. The NOS inhibitor, L-NAME, is also included for this purpose. This assay can be used to directly examine the effect of drugs, cytokines, or other experimental agents on NO production in cultured cells.