1. Intravenous injection of anti‐angiotensin II causes a transient fall in the mean blood pressure ofanaesthetized normal rats, showing that the angiotensin content in the blood plays a role for the normal blood pressure level. The subsequent return of the blood pressure to its initial level is probably caused by the cardio‐accelerator and pressor reflexes. The function of the baroreceptors is also thought to be the cause why a depressor effect of anti‐angiotensin is lacking inconscious normalrats. Lack of effect onanaesthetized nephrectomizedrats conforms to the lack of angiotensin in their blood. The somewhat increased depressor response to anti‐angiotensin in rats withacute severe constrictionof the renal arteries and the still more pronounced response in many (but not all)renal hypertensiverats conforms to increased plasma angiotensin in these rats. The somewhat decreased depressor response of ureterligated rats corresponds to their decreased sensitivity to angiotensin and renin. 2. Anti‐angiotensin causes adecreased response to injected angiotensinboth in conscious normal and anaesthetized normal, nephrectomized, ureterligated and renal hypertensive rats, the sensitivity in several cases being reduced to less than 1 per cent. If angiotensin iscontinuously infusedin doses of 9 ng/min, anti‐angiotensin will cause a fall of the blood pressure which in one half of the cases reaches the initial level, in the other half stops at a level above the initial, even when a second dose of anti‐angiotensin is injected. When anti‐angiotensin is given between two periods of continuous infusion of angiotensin, the second rise in blood pressure will be smaller, being up to 50 per cent of the first. 3. Anti‐angiotensin causes adecreased response to renin, but the above‐mentioned groups of rats do not react in the same way. In anaesthetizednormalrats the sensitivity is reduced to about 5 to 10 per cent.Nephrectomizedandureterligatedrats are still able to react with a pressor response to doses of renin which are without effect on the blood pressure of anti‐angiotensin pretreated normal rats. The maximum response of rats belonging in these two groups is, however, only up to 50 per cent of that found in non pretreated nephrectomized or ureterligated rats. If given shortlyafterrenin, anti‐angiotensin injection will cause a rapid fall in the blood pressure to the initial level in normal and ureterligated rats, while there is only a partial return to a level markedly above the initial