Enzymatic as well as nonenzymatic mechanisms generate nitric oxide (NO) from nitrite in the blood, stomach, saliva, and urine, as well as in the skin. In human skin, the inorganic anions nitrate and nitrite are not inert end products of endogenous nitric oxide metabolism, as long believed, but can be recycled by environmental stimuli such as UVA radiation to form NO; thus, they represent an important alternative nonenzymatic physiological source of NO.