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Selective breedings of male and female spontaneously hypertensive rats (OKAMOTO and AOKI) which showed persistent hypertension (blood pressure exceeding 150 mm.Hg) for over a month were repeated successively from F5 rats in the same manner as mentioned in the previous report and rats from F6 to F&glt;12> have been obtained up to the present time. Chronological observations were performed on the body weight and blood pressure of a total of 740 spontaneously hypertensive rats, as well as of 60 control rats of the Wistar strain. Both body weight and blood pressure (by tail-water plethysmographic method) were measured once a week from 5 weeks of age through 60 weeks and the following results were obtained. 1. The body weight of the spontaneously hypertensive rats showed no significant difference as compared with that of the controls. 2. The blood pressure of the control rats was almost stationary after 10 weeks of age and never exceeded 140 mm.Hg throughout 60 weeks either in males or females. The increase in blood pressure of the spontaneously hypertensive rats showed no difference among the seven generations examined. The average systolic blood pressure of the spontaneously hypertensive rats rose with age and was significantly higher than that of the controls of the same ages after 10 weeks of age among all rats except the F9 females, in which it was after 15 weeks. The levels of the average systolic blood pressure were about 190 mm.Hg in males and 180 mm.Hg in females at 30 weeks and after. 3. Some (6.7-10.0 per cent) of the control rats showed spontaneous hypertension with blood pressure levels not exceeding 160 mm. Hg after 15 weeks of age. In the spontaneously hypertensive rats development of spontaneous hypertension was observed even at 5 weeks of age and all animals had become hypertensive within 20 weeks. Severe hypertension with the blood pressure exceeding 200 mm.Hg began to be found at 10 weeks of age and its frequency averaged about 50 per cent in males and 30 per cent in females at 30 weeks of age. 4. The blood pressure of the male spontaneously hypertensive rats was about 12.2 mm.Hg above that of the females on the average. The males tended to develop into hypertension at a lower age than the females. 5. The coures of the blood pressure seemed to be fixed in the spontaneously hypertensive rats following F6, and several stages were distinguished in relation to the blood pressure as follows. Namely, the period up to 6-7 weeks of age was referred to as the "prehypertensive stage", from 7 to 20 weeks as the "transitional stage of hypertension", from 20 to 25 weeks as the "initial stage of hypertension" and after 25 weeks as the "advanced stage of hypertension". 6. The spontaneous hypertension of rats seems to have characteristics similar to human essential hypertension in certain aspects, but identification of hypertension in the spontaneously hypertensive rats and human essential hypertension is a question for further study, and may be related to elucidation of the pathogenesis of essential hypertension.