Systemic arterial compliance was measured in 22 patients with permanent essential hypertension and compared with 11 sex- and age-matched normal normal subjects. Determinations were made from analysis of the monoexponential blood pressure-time curve during diastole, according to a simple visco-elastic model. Arterial compliance was significantly decreased (P less than 0.001) in hypertensives. In the overall population, arterial compliance was negatively correlated to age (P less than 0.005) and blood pressure (P less than 0.001), suggesting that the changes in compliance could be attributable to the level of blood pressure per se and/or to the rigidity of the arterial wall. Administration of vasoactive substances (angiotensin and sodium nitroprusside) enabled a strong negative relationship (P less than 0.01) between arterial compliance and diastolic blood pressure to be demonstrated in each individual. The slope of the curve was not dependent on age and represented the ability to decrease compliance per unit rise in pressure. The slope was steeper in hypertensives, suggesting a change reactivity of the arterial wall in these patients.