Aim of the study was to reveal the possible factors regulating plasma endothelin (ET) levels in vivo in patients with essential hypertension (EH) by the simultaneous determination of plasma renin activity (PRA) and plasma aldosterone (ALD). In addition, the possible relationship between ET and circulating endothelial cells as a marker of endothelial damage was also investigated. The postural test revealed a significant increase of ET levels (26.7 +/- 9 vs 11.5 +/- 3 fmol/ml, p < 0.05) in the upright position. Captopril administration did not change plasma ET levels. No significant correlation was found between ET and PRA or ALD. Although a tendency to a positive correlation between ET and circulating endothelial cells (as the marker of endothelial perturbation) was found, it did not attain statistical significance. Our data do not support the suggestion that the renin-angiotensin-aldosterone system plays a major role in the regulation of ET secretion in vivo in EH. Postural stimulation of ET secretion may be caused by other factors than renin-angiotensin-aldosterone system.