Embryonic blood vessels have not been grown in organ culture, in a way which might easily submit them to studies of vascular organogenesis. The chick embryo dorsal aorta is easily accessible and relatively simple to explant to culture. Its organ culture may provide a model for wounding and repair of the intima and/or media and provide a model for studies of growth (or maintenance) and differentiation. Fresh dorsal aorta and its organ culture from the chick embryo was characterized morphologically by light microscopy scanning (SEM) and transmission electron microscopy (TEM) from days 4 to 18. Explants were incubated up to 4 days at 37.5 degrees C in normal atmosphere in media including whole chick embryo extract (EE) diluted to 20%. The normal fine structure by SEM shows that as the embryo ages the endothelial cells of the aorta tend to elongate, their microvilli lengthen and increase in size and number up to about 11 days, after which they decrease. TEM also demonstrates an increased differentiation of smooth muscle cells as embryonic age advances. The best culture medium was EE20-Tyrode's. Most of the explants from 14 and 17 days cultured in EE20-Tyrode's contracted into small balls of tissue, endothelial side out. None of these explants showed any significant cellular degeneration. None of the explants survived in EE20-DMEM. Addition of insulin to each medium had no effect on the explants. Intimal cells resembling myofibroblasts were observed in the contracted explants. Using a simple culture condition the chick embryo dorsal aorta can be sustained in organ culture for at least 4 days. The most suitable age for organ culture studies is 9 days (or perhaps somewhat younger); however, it is the most difficult to obtain.