Different research areas might gain from the use of cultured endothelial cells. An understanding of how endothelial cells interact with hormones, plasma constituents and drugs, could make a fresh contribution to knowledge of the functional and pharmacological responses of different organs. In vitro cultures of EC are an easy tool for these studies. The culture medium can be artificially modified and the biological responses monitored. This technique, however, still presents limitations. These are: the relatively few cells that can be obtained; the limited number of vascular districts that can be used as cell sources; and the functional modifications of the cells when kept in tissue culture.