Though only recently uncovered as a physiologic messenger, nitric oxide (NO) is increasingly appreciated as a major regulator in the nervous, immune, and cardiovascular systems. Besides mediating normal functions, NO has been implicated in pathophysiologic states as diverse as septic shock, hypertension, stroke, and neurodegenerative diseases. Biological roles for NO were first established in inflammatory responses and blood vessel reactivity. Studies of nitrosamines as carcinogens demon strated the existence of endogenous nitrates, since germ-free rats excrete large amounts of nitrates as do humans, whose excretion rises markedly