SYNOPSISThe morphology and physical ultrastructure of the lipoprotein component of elastic tissue has been characterized; its synthesis by the interaction of elastin and lipid by van der Waals' forces was observed. Comparative studies of the elastic tissue of chicken and young human aorta have been made.In vivostudies demonstrate how general lipid metabolism, and morphological and histochemical changes are integrated. The general lipid metabolism and that which occurs in vascular tissuein situwere studied in chickens which received a hypercholesterolæmic diet, and a prophylactic therapy with the enzyme E1*** of the elastase complex and a globulin fraction of the serum. Physiological adaption to the diet was shown, especially by the formation of elastin lipoprotein in the vascular wall, which thereby demonstrates a mode of enzyme action.Macromolecular reactions have been visualized with the electron microscope in the vascular wall, especially in areas where compression occurs.The activity of fibroblasts of the internal elastic lamina of the chicken has been observed and the intracellular formation and secretion of sheet elastin with retractile properties is described for the first time; filaments have also been observed