The tunica media of thoracic aortas from female rats in age from newborn to 12 weeks were analyzed quantitatively by using stereologic techniques in the electron microscope. Collagen, elastin, smooth muscle, myofilaments, Golgi apparatus, ergastoplasm, and surface to volume ratios were among those components quantified. These parameters were correlated with measurements of medial thickness, blood pressure, and tangenital tension in the wall and with biochemical estimates of collagen and elastin. A progressive increase in morphologically recognizable collagen in the media was in parallel with the increasing tangenital wall tension. Biochemical analysis indicated the presence of much soluble collagen in the early stages of development. Elastin was the major component contributing to increasing wall thickness. The elastin laminae were completely formed by 4 weeks of age, but further elastin was laid down as branches extending between medial cells in the aortas of animals between 2 and 12 weeks of age. Smooth muscle cells were ovoid in young animals and contained large amounts of ergastoplasma and Golgi apparatus, but very few myofilaments. With advancing age, these cells became irregular in outline, and their surface to volume ratios doubled. The myofilament volume increased greatly during development, associated with a decrease in" undifferentiated" cytoplasm, ergastoplasm, and Golgi apparatus. The maximal development of the latter two organelles was coincident with large increases in the connective tissue components.