This study was designed to determine plasma endothelin-1 levels in patients with essential hypertension and diabetes mellitus. Endothelin immunoreactivity was measured in normal controls (n = 30), mild-moderate essential hypertensives (n = 25), Type II diabetic normotensives (n = 25) and hypertensive patients (n = 20). In addition, in ten patients of each group we investigated the relationships of endothelin with other vasoactive hormones. Plasma endothelin concentrations were similar in healthy controls, in essential hypertensives and in diabetic patients with or without hypertension, averaging 8.23 +/- 1.68 pg/ml, 7.7 +/- 1.1 pg/ml, 5.05 +/- 0.94 pg/ml, 7.88 +/- 1.41 pg/ml, respectively. No correlations were observed between endothelin and concentrations of plasma renin activity, aldosterone, catecholamines, atrial natriuretic peptide and arginine-vasopressin. The present study suggests that Type II diabetic patients with or without essential hypertension do not have demonstrably higher values of plasma endothelin than essential hypertensives or healthy subjects.