In response to biochemical factors like catecholamines, bradykinins, histamine and physical factors like shear stress, endothelial cells release a non prostanoid factor, called endothelium derived relaxing factor (EDRF), which relaxes vascular smooth muscle. Since this discovery in 1980, many additional agents have been shown to stimulate release of EDRF from endothelium. Biological and chemical evidence has supported the proposal that EDRF is actually nitric oxide (NO). Research on the synthesis, inhibition and physiological roles of nitric oxide (NO) has led to studies of its involvement in blood pressure homeostasis and immune functions.