1. Infusion of sufficient renin to raise the blood pressure of normal rats to hypertensive levels resulted in increased renin in the arterial wall.
2. Arterial wall renin and renal venous renin were normal in younger spontaneously hypertensive rats, but in older spontaneously hypertensive rats arterial wall renin was significantly increased and renal venous renin was significantly decreased.
3. Arterial wall renin in rats with either acute or chronic two-kidney Goldblatt renal hypertension was significantly increased, whereas circulatory renin was elevated in the former, but depressed in the latter.
4. Arterial wall renin may play a role in the maintenance of acute and chronic renal hypertension and also perhaps of spontaneous hypertension of long duration in older rats.