Nitric oxide (NO) produced by the action of NO synthase on arginine plays many important roles in physiological and pathophysiological processes as well as in the maintenance of neurotransmission, vascular regulation and immunity systems. NO is formed in small amounts in vivo and is rapidly decomposed by interaction with oxygen, making its measurement difficult. NO is usually determined by chemiluminescence, EPR, electrochemically and by the Griess assay.