The discovery that nitrogen monoxide or nitric oxide (NO)is a biologically produced free radical has revolutionized our thinking about physiological and pathological processes. This discovery has ignited enormous interest in the scientific community. When generated at low levels, NO is a signaling molecule, but at high concentration, NO is a cytotoxic molecule. The physiological and pathological processes of NO production and metabolism and its targets, currently areas of intensive research, have important pharmacologic implications for health and disease.