From renin measurements made in blood collected simultaneously from renal veins, aorta and vena cava, an equation was developed for estimating renin secretion rates in patients with three renin subtypes of essential hypertension. From these data a second equation was derived for estimating differential renal plasma flow in patients with unequal kidney perfusion. The latter equation achieves maximum precision when there is no renin secretion from one side. Plasma renin activity was identical in blood collected from the aorta or the vena cava. It was also similar, but higher, in blood collected from either right or left renal veins. The ratio of renin from the two renal veins, an expression of the variability in renal vein renin measurements in essential hypertension, was 1.5 or less in 87 per cent of patients and less than 1.63 in 95 per cent. Renal vein renin content remained proportional to arterial renin over the range of peripheral renin levels found in essential hypertension, so that renal vein renin concentration from each kidney was consistently 124 per cent of arterial renin. The constancy of this relationship complements previous observations indicating that the metabolic clearance rate of renin is proportional to arterial renin levels. The observed equality of renin values between renal veins suggests that differential renal plasma flow is fairly equal and constant in patients with essential hypertension. Moreover, since renal plasma flow from each kidney is inversely related to the increment in renal vein renin concentration relative to arterial renin input [(V-A)/A], differential changes in (V-A)/A can be used to identify differential changes in renal plasma flow. These derived interrelationships are relevant to an analysis of renovascular hypertension since, with this approach, reductions in renal plasma flow can be estimated using only renal vein and arterial renin measurements and adequacy of sampling can be assessed from the sum of (V-A)/A from each kidney. There was no measurable difference in plasma renin substrate in the three renin subgroups of patients with essential hypertension so that observed differences in plasma renin activity levels appear entirely due to differences in renal renin secretion. Under conditions of this study renal renin secretion per minute was 144 times the arterial renin level.