Plasma renin activity (PRA) and angiotensin II (AII) were measured in peripheral venous samples obtained from the following subgroups: normal ambulant subjects (n = 61): normal subjects after 7 days treatment with a thiazide diuretic (n = 27): hypertensive subjects before and after a thiazide diruretic (n = 35); severely hypertensive subjects after diazoxide treatment (n = 14) and untreated severely hypertensive subjects (n = 27). In addition aortic (n = 27) and renal vein (n =48) samples were taken from the latter group during investigation of possible renovascular hypertension. Values for PRA and AII were clearly correlated in all groups. Further, calculated regression lines of PRA upon AII did not differ significantly from that calculated for normal subjects, with the exception of the intercept of the diazoxide treated group. However, when the regression line for renal venous samples was compared with that calculated for peripheral venous samples of all other subjects (199), there was a relative increase in the ratio of PRA to AII at all values. Thus, except in the renal vein, measurement of AII and PRA appear to reflect changes in activity of the renin-angiotensin system equally well and there is no support for the view that there is a discrepancy between measured AII and PRA in peripheral venous blood in the clinical situations examined here.