Blood pressure, renal blood flow, total blood volume (TBV), plasma renin activity, and vascular reactivities to angiotensin and norepinephrine were studied in 48 normotensive subjects and 106 essential, sustained, hypertensive patients with normal renal function, balanced sodium intake, and urinary output. A significant negative pressure-volume relationship was observed in normal subjects. Among the hypertensive patients, some were inside the 95% confidence limits of the normal curve and the others were above, indicating a disturbance in the pressure-volume relationship. A quantitative evaluation of the pressure-volume disturbance was proposed and discussed. The blood pressure of each hypertensive patient corresponded to two different blood volume values: the renal valve and the theoretical value extrapolated from the normal curve. The difference between the two values was called deltaTBV and was positive in hypertensive patients. The deltaTBV value was negatively correlated with the renal blood flow, the creatinine clearance, the plasma renin activity, and the vascular reactivities to angiotensin and norepinephrine (P less than 0.0001). The parameters were not correlated with the real blood volume. This study demonstrates quantitatively a pressure-volume disturbance in essential hypertension. This disturbance is strongly correlated with the renal function and the renin-angiotensin system changes.