Our objective was to investigate the changes in renal hemodynamics, and the role of these changes in the pathogenesis of essential hypertension. We used a thermodilution renal vein catheter and measured left renal blood flow in 21 patients with essential hypertension. Arterial pressure was monitored concurrently. Cardiac output was measured in 11 patients to calculate total peripheral vascular resistance. Systolic, diastolic, and mean arterial pressure were 187 +/- 7 mmHg, 97 +/- 5 mmHg and 127 +/- 5 mmHg, respectively. Left renal blood flow was 8.01 +/- 0.38 ml/s. Left renal vascular resistance, calculated as mean arterial pressure divided by left renal blood flow, was 16.4 +/- 0.8 mmHg ml-1s, and was positively correlated with systolic, diastolic and mean arterial pressures (p < 0.05). However, no significant correlation was observed between arterial pressure and left renal blood flow. Plasma renin activity in left renal venous blood, measured in 15 patients, was not significantly correlated with renal hemodynamics. Cardiac output and total peripheral vascular resistance were 81.4 +/- 6.0 l/min and 1.68 +/- 0.18 mmHg ml-1s, respectively. Total peripheral vascular resistance was not significantly correlated with either left renal blood flow or renal vascular resistance. The data suggest that a rise in renal vascular resistance may parallel the rise in blood pressure, but not that of total peripheral vascular resistance.