Infusion of the angiotensin II inhibitor 1-Sar-8-Ala angiotensin II causes a hypotensive effect in anaesthetized rats but not in conscious normal rats; it lowers the blood pressure both in anaesthetized and in conscious rats with renal hypertension due to clamping of one renal artery, no matter whether the opposite kidney remains intact or it has been removed. In the latter type of hypertension, higher doses are apparently needed. These findings indicate that the renin-angiotensin system plays a role both for the maintenance of the normal blood pressure and in the pathogenesis of both types of renal hypertension. Infusion of the inhibitor results in a primary transient, short pressor response, which in nephrectomized animals is smaller than that in normal rats, being even smaller or totally absent, in renal hypertensive rats.