Effects of a dietary intake of the polyunsaturated omega-6 essential fatty acids (EFAs) linoleic and gamma-linolenic acids (GLA) on blood lipids, platelet function, and vascular prostacyclin production were studied 12 hyperlipidemic patients (doses of 3 g/day) and 12 male Wistar rats (doses of 3 mg/kg/day) for 4 months. In humans, GLA supplementation decreased plasma triglyceride (TG) levels by 48% (p < 0.001) and increased HDL-cholesterol concentration by 22% (p < 0.01). Total cholesterol and LDL-cholesterol levels were significantly decreased by omega-6 EFAs. Platelet aggregation induced by low concentrations of adenosine diphosphate (ADP) and epinephrine, and serum thromboxane B2 decreased by 45% both in humans and animals after GLA supplementation. Bleeding time increased 40% (p , 0.01). In rats, vascular prostacyclin production measured by radioimmunoassay of 6-keto-PGF1 alpha was enhanced by GLA intake. These effects of omega-6 EFAs may contribute to cardiovascular protection and prevention of the atherosclerotic disease.