By breeding and feeding salt to spontaneously hypertensive rats (SHR) continuously over a long period (until 60 wk old), rats with systolic blood pressures (SBP) of over 270 mmHg were prepared. It was studied whether or not supplying large amounts of vitamin C (200 mg/rat/d) over this period might bring any beneficial effect to blood pressure. Moreover, physico-chemical studies were performed to measure the components and enzymes in the blood and urine at 53 and 60 wk-old, and biochemical studies on vitamin C were also carried out in this experiment. Male (14 rats: 7 wk-old, 100-105 g) and female (15 rats: 7 wk-old, 95-100 g) SHR were divided into three groups and bred continuously for 53 wk. The A group rats were given salt (2.5 g/100 g of diet), the B group rats were given salt and vitamin C (500 mg/100 mL of drinking water), and the C group rats were controls. The results showed almost the same tendencies between male and female rats. The body weights of the SHR in groups A and B were slightly lower than group C. The amount of food intake in groups A and B was almost the same as group C. The amount of water intake was, in the order from highest to lowest, group A, B and C. The SBP of group A rats exhibited the highest value among the three groups. The SBP of group B rats given vitamin C simultaneously with the salt resulted in a low blood pressure level close to that of the controls (group C). Furthermore, the DBP (diastolic blood pressure) also reflected the antihypertensive effect of vitamin C as well. The heartbeat of the rats was highest in group A, and was comparable to the value in the rats receiving vitamin C simultaneously with salt. For the tests on occult blood and protein in the urine, group A rats showed strong positive reactions, whereas the group B and C rats had decreased results for both tests. The organ weights of the liver, stomach, spleen, adrenal gland and kidneys per 100 g rat body weight were not different among the three groups. The values for the bilirubin content, and the enzyme activities of ALT and AST in the blood showed to be the highest in the male rats of group A. The values from the group B rats decreased near to the normal value like the control group. Vitamin C was found to decrease the blood pressure in SHR, and also to work effectively to protect liver and kidney functions even under the condition of very high blood pressure, as high as 250 mmHg.