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Plasma concentration of two main cardiovascular substances - atrial natriuretic factor (ANF) and endothelin - were studied in control subjects (n = 21) under basal conditions and 90 minutes after oral administration of glucose. In hypertensive patients (n = 21) these determinations were repeated after 12 weeks treatment with an angiotensin I-converting enzyme inhibitor lisinopril (Prinivil, Merck Sharp and Dohme). While basal and post-glucose ANF concentrations did not differ in controls and hypertensive patients, a tendency to the higher endothelin levels was found in our group of essential hypertension when compared to normotensive subjects. Glucose loading did not change significantly ANF concentrations in any studied group but significantly lowered plasma endothelin in both controls (from 13 +/- 0.95 to 9.50 +/- 0.95 fmol/ml) and hypertensive patients (from 15.05 +/- 1.23 to 12.15 +/- 1.03 fmol/min). Treatment of hypertensive patients with lisinopril paradoxically increased concentrations of ANF (from 6.43 +/- 2.53 to 11.47 +/- 4.90 fmol/ml) and lowered that of endothelin (from 15.05 +/- 1.23 to 12.17 +/- 1.58 fmol/ml). From our findings we may suggest that the relative predominance of the vasoconstrictor (endothelin) over the vasodilator (ANF) humoral substances might participate in pathogenesis of EH and that the reversal of this disadvantageous ratio after lisinopril (increase of ANF and decrease of endothelin) might contribute to the blood pressure reducing effect of ACEI. The drop in plasma endothelin after glucose remains so far unexplained consequence of glucose loading in both control and hypertensive subjects.