Daily changes of cultured endothelial cells were observed by phase contrast microscopy, time-lapse microcinematography and electron microscopy. By light microscopy, the cultured cells are polygonal in shape and form monolayered clusters. However, they become elongated like fibroblasts between 10 and 14 days. Proliferation of the cultured cells by mitotic division could not be recognized by cinematographic observation. On the contrary, many binucleate cells occurred in the periphery of the cluster throughout the proliferation period, suggesting that the cells proliferate here, at least predominantly, by amitotic division. By electron microscopy, specific endothelial granules and two types of filaments (about 120 A and 65 A in diameter) were observed in the cultured endothelial cells. The combined cinematographic and electron microscopic observations suggested that the thiner filaments may be contractile elements involved in the movement of the cultured cells.