The most well-characterized biological function of nitric oxide (.NO) is the elevation of cyclic guanosine monophosphate (cGMP) levels by the activation of guanylyl cyclase [1-3]. Simplistically, this pathway involves diffusion of •NO from its site of synthesis to a target enzyme in an adjacent cell or tissue. In this respect, •NO is acting as a conventional paracrine messenger. However, the ... [Show full abstract] known chemistry of •NO suggests that this journey is not at all simple and that there is the potential for a plethora of side reactions to occur. The chemical reactions between •NO and other biological molecules impact not only on the efficiency of guanylyl cyclase activation but also contribute to the cytotoxic potential of activated immune cells. Moreover, if uncontrolled, •NO synthesis may be responsible for a variety of pathological processes.