Nitric oxide (NO) has been found to play an important role as a signal molecule in many parts of the organism. However, under certain conditions, like hypoxia or ischemia, it acts as a cytotoxic effector molecule. It appears paradoxical that NO on the one hand acts as a physiological intercellular messenger and on the other can have damaging effects to cells. To make things even more complicated cytoprotective properties of NO are also described. Whether NO is useful or harmful depends on the rate and location of its production.