Abstract: The plasma endothelin （ET） levels were measured by radioimmunoassay in 30 patients with essential hypertension before and after treatment by enalapril. The resultsshowed the plasma ET level in patients before treatment wassignificantly higher than that in controls （P＜0. 001 ）. A signiffornt positive correlation was found between ET level and arterial blood pressure. The plasma ET level decreased significantly after three weeks of enalapril treatment （P ＜0. 001 ）, butstill higber than that in controls （P＜0. 01 ）. The results indicate that ET may play an important role in the pathogenesis ofessential hyPertension and enalapril can reduce the plasma ETlevel significantly.