Recent studies showed that supplementation of a large dose of fish oil is effective in lowering blood pressure. Whether supplementation of a low dose of fish oil is also effective in lowering blood pressure is controversial. The present study evaluates the hemodynamic and humoral effects of low-dose dietary fish oil supplementation in patients with mild hypertension. Eleven patients with mild hypertension were given 3 g/day of n-3 polyunsaturated fatty acids for 6 week. Twenty-four hour blood pressure monitoring and humoral parameters were recorded before and during fish oil treatment. In five patients, blood pressure response to angiotensin II (AII) infusion (1, 2, 3, 6, 9 ng/kg/min) was also recorded before and during treatment. Casual mean arterial pressure was unchanged (111 +/- 1 mm Hg v 109 +/- 3 mm Hg; P = NS). Average 24-h mean arterial pressure (MAP) also did not change during fish oil treatment (98 +/- 2 mm Hg v 99 +/- 3 mm Hg; P = NS). Fish oil supplementation did not attenuate the vascular reactivity to AII infusion. Maximal MAP following AII infusion (9 ng/kg/min) was 128 +/- 5 mm Hg before and 129 +/- 7 mm Hg during fish oil treatment (P = NS). Plasma levels of norepinephrine, renin activity, aldosterone, and atrial natriuretic peptide remained unchanged during treatment. Plasma levels of total cholesterol slightly increased from 200 +/- 10 mg/dL to 211 +/- 9 mg/dL (P < .05), but plasma levels of high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, and triglycerides were unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)