NO is obviously identical with the relaxation factor produced by the vascular endothelium (EDRF) and is also the substance responsible for some other biological activities. It is formed in the organism from L-arginine by the action of the enzyme NO synthetase. The main mechanism of action is the activation of the enzyme guanyl cyclase and the result is an increase of the intracellular level of cyclic guanyl monophosphate. Depending on the type of effector cell, either vasodilatation occurs and adhesion is inhibited and the blood platelets coagulate or the cytotoxicity of macrophages increases. With the development of new, more effective inhibitors of NO synthetase there is also the possibility to study the physiological importance of NO in more detail. These new discoveries provide a more profound biochemical and pharmacological basis and perhaps also new indications or preventive possibilities of the known treatment of vascular spasms by nitroderivatives; moreover, there is the possibility to seek new ways in the anti-tumourous and antimicrobial treatment and elsewhere.