The calf's aorta, has been maintained in vitro for prolonged periods under aseptic conditions. Studying the aorta's function as regards cholesterol biosynthesis, accumulation and release, under the experimental conditions of hyper and normal blood pressure, estrone administration, and dilutions of serum have led to the conclusion that cholesterol metabolism can be markedly altered by these conditions and that there may be factors or systems normally present in blood which regulate lipid metabolism in the organ.