Many investigators are studying on the point that cholesterols and fats are greatly involved in the causative mechanism of arteriosclerosis. However, even in the case of an advanced arteriosclerosis cholesterols in blood are not necessarily increased, but quite many show the cholesterol level within the normal range. Furthermore, subsequent research works indicate the problems such at the ratio of cholesterols against phospho-lipids and chylomicron, as well as studies on lipoproteins and electropaper-chromatography are carried out, but conclusive causative factors still remain to be discovered. As reported in Parts 1 and 2 the author recognized the occurrence of arteriosclerosis of the so-called 'adrenalin-injection type' (Monckeberg type) in the aorta of the rabbits injected with cholesterol emulsion or administered with a large amount of cholesterol for consecutive days, and also the occurrence of arteriosclerosis of the so-called 'cholesterol type (atheroma type) in the rabbits orally adminstered with a small amount of cholesterol for sucessive days. Thus the author pointed out that what was believed to be two types of arteriosclerosis caused by two entirely different factors was in fact the arteriosclerosis induced by the same abnormality in the fat metabolism, and deduced that the differences observable in this disease arose from the differences in the chemical constitution of cholesterols in the blood. In the present report an attempt has been made to clarify further the actual characteristics in the occurrence of two kinds of arteriosclerosis by observing what differences in the chemical constitution of fats in the blood would be brought about by different methods of administering cholesterols on one hand and also by studying the changes in cholesterol contents in the blood at the time of nor-adrenalin injection into rabbits on the other, Using 8 rabbits, the present experiments were conducted in three groups; namely, a group (2 rabbits) given oral administration of 0.7 g. cholesterol daily; another group (2 rabbits) given daily intravenous injection of 10 cc. 1% cholesterol emulsion; and still other group (3 rabbits) given daily intravenous injection of 1 cc. 0.1% adrenalin; and at a fixed interval after the adminstration, blood was drawn from the auricular vein, and the total amount of cholesterol and the free cholesterol content were determined by the digitonin method. After weighing these results in conjuction with the histological or cytochemical findings of Parts 1 and 2, the author arrived at the following conclusions: 1. The intravenous injection of cholesterol emulsion the same as the nor-adrenalin injection induces a transient and rapid rise in free cholesterols in the blood and the degeneration of muscle fibers or calcium deposit in the artery. 2. In the case of oral adminstration of cholesterol the total amount of cholesterols increses step by step, but the ratio of the free-cholesterol content against the total amount of cholesterol in creases always within the normal range, showing the foam cell formation in visceral tissues as well as atheroma degeneration under the inner coat of artery in this instance. 3. From these results it is velieved that the rise in the free cholesterol content of blood acts as an important factor causing the degeneration or the necrosis of the media of the artery, developing to the Monckebergs median necrosis of artery, while in the case of the rise in the cholesterol-ester, the cholesterol ester is absorbed by the interstitial cells and disposed, forming a foam cell layer without inducing cell degeneration; and when such an increment is great, the atheroma degeneration of the aorta is brought about.