1. When the renin-angiotensin system of rats had been suppressed by a high salt diet or by bilateral nephrectomy, large doses of angiotensin II antiserum were required to block the pressor action of exogenous angiotensin II. Infusion of renin profoundly lowered the blocking requirement of such animals.
2. It is postulated that renin bound to blood vessels generates angiotensin locally which is taken up by vascular receptors. Where such receptors are left unoccupied and free to bind exogenous angiotensin, high doses of blocking antisera are required.
3. Animals with hypertension produced by renal artery constriction with contralateral nephrectomy were shown to be in positive sodium balance. Nevertheless their blocking requirement was low.
4. It is suggested that the local generation of angiotensin may play a role in the production of renal hypertension and that this accounts for the development of hypertension even in animals immunized against angiotensin.