Abstract
Nitric oxide (NO) is a naturally-occurring free radical that exists as a colorless and odorless gas. In biological solutions, NO is highly diffusible both in water and through biological membranes, with a half-life measured in seconds. Prior to the recognition that NO was the same molecule as an endothelium-derived relaxing factor some 20 years ago it was considered to be mainly an environmental pollutant. However, NO is now recognized as an endogenously produced vasodilator that modulates vascular tone and, thereby, regulates systemic and pulmonary blood flow. It has also been found to have diverse roles throughout mammalian physiology, including neurotransmission and host defense.