Effects of n-3 fatty acids in essential hypertension. We examined the effects on blood pressure, plasma lipoproteins, and platelet function when marine oil supplements (rich in n-3 fatty acids) or vegetable oil supplements (rich in n-6 fatty acids) were added to the usual diets of patients with mild essential hypertension. In a randomized, double-blind, parallel-group study, patients received 50 g of either marine oil (n = 8) or vegetable oil (n = 8) daily for 6 weeks following a baseline observation period. Diastolic blood pressure declined during treatment with fish oil (mean +/- SEM, 96 +/- 2 v 89 +/- 2 mm Hg, P = .02), but did not change with vegetable oil (92 +/- 1 v 94 +/- 1 mm Hg). Systolic blood pressure did not change significantly during either treatment. Serum triglycerides declined (by approximately 30%) in patients receiving only marine oil, but total cholesterol, LDL-, HDL-, HDL2-, and HDL3-cholesterol-subfractions and apolipoproteins A-I and B were unchanged in both treatment groups. Bleeding time increased by 33% during treatment with marine oil but did not change with vegetable oil supplements. Marine oil did not alter in vitro platelet aggregation thresholds. The lack of a significant correlation between blood pressure changes and platelet membrane fluidity, plasma renin activity, aldosterone, norepinephrine, or epinephrine suggests that these variables did not mediate the antihypertensive effect of the marine oil. We conclude that large doses of marine oil reduce diastolic blood pressure, lower triglycerides, and increase bleeding time in patients with mild hypertension. 