The hyperreactive arterial endothelial cells have been introduced in this paper. They are characterized by their ability to transport particles too large for the small holes of the internal elastic lamina locating underneath the endothelial cells, such as carbon particles with the similar size of LDL, floating beta-lipoprotein, Lp(a) and especially of VLDL, into the subendothelial space from the blood stream by their abnormally strong contracting and phagocytosis-like activity. Large particles such as carbon particles with a size of 200 to 700 angstrom are too large to penetrate further through holes of the internal elastic lamina from the subendothelial space to muscular layers of the arterial wall, resulting in stagnating for a long time in the subendothelial space, thus showing the atherogenic property of the hyperreactive arterial endothelial cells. Such endothelial cells appear spotty and streaky in the localized endothelial lining of predominantly susceptible parts to atherosclerosis in susceptible animal species such as rabbit, chicken, and rhesus monkey especially densely in their atheromatous lesions, but do not generally appear in non-susceptible animals to atherosclerosis like rate and dog. They are extremely few in infant rabbit, but increase by age.They appear in hypertensive rat, showing a characteristic distribution even in small groups of arteriessuch as the circle of Willis. Cyclic AMP, and especially dibutyryl cyclic AMP, exhibited an inhibitory effect on the hyperreactivity of those cells. Cyclic AMP phosphodiesterase inhibitors, EG467 and eg626, exhibited a powerful inhibitory effect on the contracting and phagocytosis-like activity of those cells, as in the case of pyridinolcarbamate, which enhances enzymes to produce ATP and inhibits slightly cyclic AMP phosphodiesterase, although its inhibitory effect on cyclic AMP phosphodiesterase is weaker than that of EF467 and EG626. The usefulness of the inhibitors on cyclic AMP phospodiesterase of arterial endothelial cells and platelets and on that of brain, such as EG467 AND EG626, has been suggested in the treatment of atherosclerotic disorders, especially of cerebral atherosclerosis. Some of the hitherto desperate mental disability of the aged seem to be a promising target for treatment with cyclic AMP phosphodiesterase inhibitors.