1. The role of arginine—vasopressin (AVP) and of angiotensin in blood pressure control of spontaneously hypertensive rats (SH rats, stroke-prone strain) was studied.

2. In SH rats, which drank water or 1% NaCl, plasma AVP concentrations were elevated during the benign course of hypertension and increased further when the animals entered the malignant phase. Blood pressure correlated significantly with plasma AVP concentrations in SH rats on water, but not in SH rats on saline.

3. The injection of a specific AVP antiserum lowered blood pressure significantly in SH rats on water and in SH rats on saline.

4. When the correlation between blood pressure and plasma AVP of SH rats on water was compared with the respective correlation obtained during infusion of AVP into normotensive rats, a marked shift to the left became apparent, the factor of displacement amounting to more than 1000.

5. Saralasin did not affect blood pressure of SH rats on water, except for two rats with malignant hypertension. However, in SH rats on saline, saralasin lowered blood pressure significantly.

6. It is concluded that in SH rats AVP plays an important vasopressor role in blood pressure control and that sensitization to the vasopressor effect of AVP occurs in these animals. The renin—angiotensin system is significantly involved in blood pressure control of SH rats only when they are subjected to high salt intake.

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