Nitric oxide (NO) is a reactive radical that plays an important role in many key physiological functions. NO is the oxidation product of arginine by nitric oxide synthase (NOS) and is involved in host defense development, activation of regulatory proteins and direct covalent interaction with functional biomolecules. Inhibition of NOS has the potential to produce diverse biological effects, particularly in the cardiovascular system. Simple, direct and non-radioactive procedures for measuring NOS are becoming popular in research and drug discovery. This Nitric Oxide Synthase Inhibitor Assay Kit involves two steps: a NOS reaction step during which NO is produced followed by an NO detection step. Since the NO generated by NOS is rapidly oxidized to nitrite and nitrate, the NO production is measured following reduction of nitrate to nitrite using an improved Griess Method.