SummaryPlasma renin activity has been measured with Boucher’s technique. In normal adults, the mean value of renin activity is 29,2 nanogrammes of angiotensin liberated/10 ml of plasma (S.D.±6,5). In 81 cases of essential hypertension, renin activity averaged 17,4 ng/10 ml (S.D.±4,5). In 16 cases of renal artery stenosis, renin activity averaged 80,2 ng/10 ml (S.D. + 44).However, in 5 of these cases, renin activity was only at the upper limit of the normal ranges. Normal renin plasma activity in causative renal artery stenosis can be explained in three ways :1) Systemic plasma renin activity does not reflect renal secretion in tight stenosis with low renal plasma flow; in 4 cases of renal artery stenosis with paranormal renin activity in systemic blood, renin activity in venous blood was high (304 ng/10 ml); stenotic renal artery blood flow averaged in these cases 45 ml/ minute.2) Renal blood flow and renin plasma activity may be normal in the reclining subjects and abnormally increased in erect position.3) Renin activity decreases in renal artery stenosis of long duration. None of our 5 cases of renal artery hyperplasy showed plasma renin increases neither in systemic nor in renal venous blood or in erect position. In cases of causative) renal stenosis with normal plasma renin activity in the above conditions, surgical treatment fails.