One set of the circulatory drugs has the primary property of dilating the blood-vessels and of reducing the blood-pressure. This action is in large part due to a depressing action they exert on unstriped muscular fiber, particularly on the walls of the bloodvessels. These drugs are practically the exact opposites in their action to that of adrenalin and ergot. They are all nitrites, in which the NO ion is the active part of the molecule. The most important of these are amyl nitrite, nitroglycerin, sodium and potassium nitrite, erythrol tetranitrite, and mannitol hexanitrite.

In addition to their action on the blood-vessels the nitrites affect the blood. They cause the formation of methemoglobin and thus limit oxidation in the tissues.