The maturation of vascular endothelial cells in the chick chorioallantoic membrane, from 8 to 18 days after fertilization, was investigated by light and electron microscopy. Light microscopic autoradiography following administration of tritiated thymidine was used to determine the thymidine labeling index of the endothelial cell population at various stages of development. Results indicate that, prior to day 11 of incubation, endothelial cells have the morphological characteristics of immature and relatively undifferentiated cells. During this time they exhibit a high labeling index of approximately 23%. At 11 days, the labeling index decreases to 2.8%, and subsequently the cells begin to acquire structural characteristics of mature differentiated endothelium. The pattern of endothelial cell labeling suggests that, during the period of high endothelial cell mitosis, the capillary network of the growing chorioallantoic membrane is expanding by an overall proliferation of endothelial cells in existing capillaries, rather than by formation of new capillary sprouts. The immaturity of endothelial cells in the young chorioallantois, or conversely their high rate of cell division, may influence the ability of this membrane to support grafted tissue prior to day 11.