We investigated the formation of the aortic tunica media in developing chicks from Hamburger-Hamilton stage (st) 12 to 1-day after hatching. At st 12, some cells of the lateral plate and the somite adjacent to the aorta extended processes toward the aortic endothelial tube. This is the first sign of the tunica media formation of the aorta. Then, these cells detached from the lateral plate and the somite attached to the aortic endothelial tube and covered it almost entirely by st 18. At this stage, bundles of microfilaments and spots of dense material were found in the peripheral region of cells of the tunica media (media cells); suggesting the earliest differentiation of smooth muscle cells. Subsequently, the tunica media increased in the number of cell-layers, reaching approximately 20 layers at st 34 (8-day incubation). The outer layers of the tunica media consisted of undifferentiated cells, whereas typical smooth muscle cells were found within the inner 4 to 5 layers. Elastic fibers first appeared at this st 34. With there was an increase in the elastic component, the tunica media assumed the lamella structure consisting of the alternating myocytes and elastic laminae. The aortic media 1-day after hatching essentially exhibited the same mature state as seen in the adult chickens.