(1) The single administration of one gm per kg of cholesterol was again shown to produce a strong contraction showing pinches of 6.4 per cent of endothelial cells in the thoracic aorta, and the bleb formation was also statistically significantly increased. (2) The pretreatment of animals with 5mg per kg of Premarin or 10mg per kg of pyridinolcarbamate was shown to prevent significantly the appearance of the contraction of endothelial cells of the aorta. Such Table II. Summary of the Analysis of Variance (x2 [3]) evidences suggest the significance of the contraction of endothelial cells as a key mechanism in atherogenesis.