Nitric oxide (NO), the molecule of the year 1992, is gaining recognition as an important biological mediator. Its multitude of physiologic and pathophysiologic functions result from both a wide distribution of synthesis and diverse mechanisms of action. Besides its functions as a potent vasodilator and neurotransmitter, NO is important in inflammation and immunity. Both beneficial and detrimental consequences of induced synthesis have been discovered. Information is now accumulating on the regulation and function of induced NO. The recent cloning of human inducible NO cDNA should assist in defining the role of inducible NO in human physiology and pathophysiology.