In 15 patients with essential hypertension total blood flow to a single kidney (TRBF) was measured by a constant infusion indicator dilution technic which had previously been validated in dogs. Indocyanine green indicator dilution curves then were used to calculate the fraction of renal flow traversing the rapid (cortical) circulation and the blood volume in this cortical compartment. TRBF ranged from 152 to 1033 ml/min/1.73m2 per kidney and was closely correlated with cortical flow (r=0.99). The cortical fraction of TRBF ranged from 46 to 93% and varied directly with total flow (r=0.69). Noncortical flow ranged from 12 to 85 ml/min/1.73m2 per kidney and showed no correlation with TRBF. Cortical blood volume varied from 12 to 85 ml/1.73m2 and also was closely correlated with TRBF (r=0.93). These data suggest that diminished TRBF in hypertensive patients is due to reduction in cortical flow with preservation of noncortical (medullary) flow and reduction in cortical blood volume. The reduced cortical fraction of TRBF could be a factor in the renal functional abnormalities which occur in hypertension.