It has been suggested that increased oxidative stress may be both a cause as well as a consequence of hypertension. We studied the effect of oral administration of an antioxidant peptide, taurine (2 g for 15 days) on blood pressure in 14 patients with borderline hypertension in a double-blind, placebo-controlled fashion. Systolic blood pressure after two weeks treatment with taurine decreased by 2.0 +/- 0.5 mm Hg (mean +/- SE ; p less than .05 by paired t test), compared with a 0.7 +/- 0.9 mm Hg decrease (NS) after the same treatment with placebo. Diastolic blood pressure after two weeks treatment with taurine decreased by 2.1 +/- 0.7 mm Hg (p less than .05) compared with 1.1 +/- 2.9 mm Hg (NS) after the same treatment with placebo. Total antioxidant activity of plasma increased after two weeks treatment with taurine from 1.69±0.08 to 1.89±0.05 mmol/L. This trial demonstrated the beneficial effect of taurine as antioxidant on blood pressure in borderline hypertensive men who were not taking antihypertensive medication.