Nitric oxide is an important mediator of physiological and pathological processes. It is a lipophilic molecule that contains a single unpaired electron which causes NO to be chemically reactive, and to function as a free radical with a short lifetime. NO can act by direct and indirect effects. Direct effects occur between NO and specific biological molecules whereas indirect effects are mediated by reactive nitrogen oxide species (RNOS) formed from the reaction of NO either with oxygen or superoxide. This review discusses the metabolic pathways of NO.