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Pathological study of cardiovascular system of 173 animals from the spontaneously hypertensive rats (OKAMOTO and AOKI) revealed the following results. 1. In the spontaneously hypertensive rats, periarteritis nodosa was the most remarkable and frequent vascular disease (25% in contrast to 5% in normotensive control animals), and even the development of nephrosclerosis, which has never been experienced in control animals, was not rare (13%). 2. The incidence of periarteritis nodosa showed an increase in proportion to the severity and duration of hypertension. 3. Nephrosclerosis was found almost exclusively in the rats whose blood pressure level is very high (over 200mm Hg.) or in which the high blood pressure, if not so high, lasts for very long time (over 16 months). 4. As hypertension developed, heart gained in weight, and concentric cardiac hypertrophy occurred. 5. Histologically examined, in the arterioles or small arteries, definite changes corresponding to human benign arterio (lo) sclerosis were no more than thickenig of vessel walls. Contrarily, the vascular lesions similar to those in human malignant hypertension were remarkable and rich in variety, that is to say, proliferative process and fibrinoid degeneration or necrosis of the vessel wall were common. Larger arteries were involved mainly by periarteritis nodosa, sometimes intimal proliferation and thickening of the vessel walls, but no atherosclerosis was experienced. 6. In the heart, myocardial scarring was noticed. It is significant for the future studies of vascular lesions in the human essential hypertension, that in the spontaneously hypertensive rats, extensive and severe vascular lesions, nephrosclerosis and cardiac lesions develop without any artificial procedure.