Nitric oxide (NO) is a key biosignaling molecule produced in both peripheral tissues and the central nervous system by a family of enzymes known as nitric oxide synthases (NOSs). NOSs convert L-arginine to stoichiometric quantities of NO and L-citrulline using molecular oxygen and NADPH as cofactors. Techniques for measurement of NO and NOS activity are essential to demonstrate the role of NO and NO-derived species in biological systems. This unit describes two methods for detection of NO: a direct method employing chemiluminescent detection and one based on quantification of the stable oxidation products with detection using the Griess reagent. Additionally, NOS activity can be quantified by measuring the conversion of radiolabeled L-arginine to radiolabeled L-citrulline.