Several studies were performed to evaluate the degree of inhibition of angiotensin-converting enzyme (ACE) by an ACE inhibitor by assessing the blood pressure response to a continuous i.v. infusion of increasing doses of angiotensin I in healthy volunteers. We assessed pharmacokinetic and pharmacodynamic interactions of the ACE inhibitor cilazapril and the p-blocker propranolol in healthy volunteers and patients with essential hypertension. We also evaluated the effect of cilazapril on aortic compliance in hypertension by pulse-wave velocity along the aorta. We showed that single oral doses of cilazapril 4 mg, captopril 25 mg, or enalapril 10 mg shifted the angiotensin I dose-effect curve to the right and determined a pharmacologic half-life of about 4 h for cilazapril. Increasing single oral doses (1.25, 3.75, 10, and 30 mg) of cilazapril reduced diastolic blood pressure dose-dependently and shifted the angiotensin I dose-response curves to the right. The dose representing 50% inhibition of ACE activity (apparent Ki dose) was about 0.6 mg 3 h after cilazapril administration. Cilazapril and propranolol did not exhibit significant pharmacokinetic interaction in healthy volunteers; each drug reduced diastolic and systolic blood pressure by about 7 mm Hg, and this was doubled by the combination. Monotherapy with each drug reduced blood pressure, and combined administration enhanced the antihypertensive effect. The reduction in cardiac output and the increase in total peripheral resistance induced by propranolol were attenuated by the cilazapril-propranolol combination. In comparison with hydrochlorothiazide, the ACE inhibitor at the same degree of blood pressure reduction seems to have a more intense effect on aortic compliance. Therefore, ACE inhibition by cilazapril, in addition to its effects on blood pressure, could provide further benefit in hypertensive patients.