On October 12, 1998, the Nobel Assembly awarded the Nobel Prize in Medicine and Physiology to scientists Robert Furchgott, Louis Ignarro, and Ferid Murad for their discoveries concerning nitric oxide as a signalling molecule in the cardiovascular system. In contrast with the short research history of the enzymatic synthesis of NO, the introduction of nitrate-containing compounds for medicinal purposes marked its 150th anniversary in 1997. Glyceryl trinitrate (nitroglycerin; GTN) is the first compound of this category. Alfred Nobel (the founder of Nobel Prize) himself had suffered from angina pectoris and was prescribed nitroglycerin for his chest pain. Almost a century later, research in the NO field has dramatically extended and the role of NO in physiology and pathology has been extensively studied. The steady-state concentration and the biological effects of NO are critically determined not only by its rate of formation, but also by its rate of decomposition. Biotransformation of NO and its related N-oxides occurs via different metabolic routes within the body and presents another attractive field for our research as well as for the venture of drug discovery.