This study was undertaken to clarify the mechanism of the antihypertensive effect of coenzyme Q10 (CoQ10). Twenty-six patients with essential arterial hypertension were treated with oral CoQ10, 50 mg twice daily for 10 weeks. Plasma CoQ10, serum total and high-density lipoprotein (HDL) cholesterol, and blood pressure were determined in all patients before and at the end of the 10-week period. At the end of the treatment, systolic blood pressure (SBP) decreased from 164.5 +/- 3.1 to 146.7 +/- 4.1 mmHg and diastolic blood pressure (DBP) decreased from 98.1 +/- 1.7 to 86.1 +/- 1.3 mmHg (P < 0.001). Plasma CoQ10 values increased from 0.64 +/- 0.1 microgram/ml to 1.61 +/- 0.3 micrograms/ml (P < 0.02). Serum total cholesterol decreased from 222.9 +/- 13 mg/dl to 213.3 +/- 12 mg/dl (P < 0.005) and serum HDL cholesterol increased from 41.1 +/- 1.5 mg/dl to 43.1 +/- 1.5 mg/dl (P < 0.01). In a first group of 10 patients serum sodium and potassium, plasma clinostatic and orthostatic renin activity, urinary aldosterone, 24-hour sodium and potassium were determined before and at the end of the 10-week period. In five of these patients peripheral resistances were evaluated with radionuclide angiocardiography. Total peripheral resistances were 2,283 +/- 88 dyne.s.cm-5 before treatment and 1,627 +/- 158 dyn.s.cm-5 after treatment (P < 0.02). Plasma renin activity, serum and urinary sodium and potassium, and urinary aldosterone did not change. In a second group of 11 patients, plasma endothelin, electrocardiogram, two-dimensional echocardiogram and 24-hour automatic blood pressure monitoring were determined.(ABSTRACT TRUNCATED AT 250 WORDS)