The L-arginine: nitric oxide pathway is widely recognized as an important regulator of cell function and communication in a variety of physiological and pathophysiological situations. Recent advances in the biochemistry and molecular biology of nitric oxide synthases have contributed significantly to our understanding of the regulation of nitric oxide synthesis in health and disease. This pathway has been implicated in the pathogenesis of septic shock, hypertension, and atherosclerosis as well as in the antihypertensive action of converting enzyme inhibitors. Progress in this field, which spans the cardiovascular, immune, and nervous systems, has been rapid, and its full potential is yet to be realized.