1. A technique was developed to measure renin concentration in nanolitre volumes of blood.
2. The renin concentration in renal venous blood was higher than in renal arterial blood.
3. The renin concentration in blood from the efferent arteriole was less than in blood from the renal artery and renal vein.
4. Renin enters the circulation distal to the efferent arteriole.
5. The release of renin into the interstitium would allow local formation of angiotensin and the system could act as an intrarenal control mechanism.