Relaxation produced by nitrates on venous, arteriolar, and large arterial vessels is well known but has never been studied in human hypertensive aortas studied in vivo. In this investigation, the effects of acute oral administration of 20 mg of isosorbide dinitrate were evaluated in 12 patients with sustained essential hypertension and nine normotensives of the same age. Noninvasive measurements of systolic, diastolic, and mean arterial pressure, carotid-femoral pulse wave velocity and aortic-arch diastolic diameter using suprasternal echocardiography were determined before and 3 hours after drug administration. In normal subjects, isosorbid dinitrate significantly decreased systolic blood pressure and pulse pressure but did not affect diastolic and mean arterial pressure. In contrast, in hypertensives, the same dosage of isosorbid dinitrate decreased together systolic, diastolic mean, and pulse pressure. In both populations, pulse wave velocity decreased significantly whereas aortic arch diastolic diameter increased markedly. The increase was observed mainly in normal subjects. The study provided evidence that (1) both in normal subjects and hypertensives, isosorbide dinitrate caused an increase in aortic diameter together with an increase in arterial distensibility; and (2) the changes in mean arterial pressure were significant only in hypertensives, indicating that the altered vasodilator response in essential hypertension is not endothelium-mediated.