1. In renal hypertensive rats, increase in blood pressure above 180 mmHg may induce sodium and water loss, reduced growth rate, elevated haematocrit, a marked rise in plasma renin concentration, an increase in renin extractable from the clamped and the contralateral kidney and malignant nephrosclerosis of the contralateral kidney. These symptoms characterize the malignant phase of renal hypertension in rats.
2. When water was given as drinking fluid, ten of eighteen rats developed signs of malignant hypertension and malignant nephrosclerosis within 3–4 weeks. Administration of 0.9% saline instead of water induced higher blood-pressure levels, but only five of eighteen rats showed malignant nephrosclerosis. When drinking fluid was changed from water to saline shortly before or shortly after the onset of malignant hypertension, the condition improved, and in only one of twelve rats was malignant nephrosclerosis observed.
3. It is concluded that in renal hypertensive rats sodium supplements may improve or prevent signs of malignant hypertension and the development of malignant nephrosclerosis.