Plasma endothelin immunoreactivity was measured in normal controls, in mild essential hypertensive patients with no antihypertensive treatment, in moderate to severe hypertensives whose blood pressure was controlled by treatment, and in moderate to severe hypertensives whose blood pressure was not controlled by treatment. Endothelin immunoreactivity levels in plasma in the four groups were similar, but had a positive correlation with blood pressure in hypertensives although not in normotensives. The concentration in plasma of both C-terminal and N-terminal atrial natriuretic peptide also had a positive correlation with blood pressure in these hypertensive patients. We conclude that if plasma immunoreactive endothelin levels represent the result of endothelial production of endothelin, in absence of changes in catabolism of the peptide, essential hypertensive patients do not appear to have increased production of endothelin.