This brief review discusses the ways, if and when available, to examine endothelium-dependent changes in diameter in human blood vessels. It stresses the problems in ensuring proper matching between arteries (and veins) from different human sources. It briefly considers the evidencein vitrosupporting the role of endothelium-derived nitric oxide, hyperpolarizing factor(s) and contracting factors (including metabolites of arachidonic acid and endothelin). It emphasizes the difficulty in extrapolating observations obtained in isolated arteries (and veins) to the intact human circulation. The overall conclusion is that the interpretations derived from animal work apply to the human vasculature.