Think of the endothelium not only as an inert blood container but as a vast endocrine gland. It stretches over the entire vascular tree with a surface area of about 400 square metres, of which most are in the capillaries. Its weight in an adult is about 1 ·5 kg and it contains an estimated 1 ·2 trillion endothelial cells (for review see Gimbrone, 1986; Ryan, 1988). In addition to the obvious barrier and transport functions, the endothelium influences its environment by the secretion of a wide range of biologically active mediators regulating immune responses, vascular tone and coagulation. The endothelium of each organ could therefore be considered to be an endocrine gland in its own right responding to external stimuli by the production of paracrine hormones and growth factors which act on neighbouring smooth muscle cells, monocytes, macrophages, fibroblasts and organ specific cells.