ABSTRACTRecent studies showed that taurine, a sulphonic amino acid, could decrease blood pressure and increase sympathoadrenal tone in DoCA-salt-treated hypertensive rats. To determine whether taurine exerts its antihypertensive action in man in a similar fashion, we studied the effect of oral administration of taurine (6 g for 7 days) on blood pressure and plasma catecholamines in 19 young patients with borderline hypertension in a double-blind, placebo-controlled fashion. Systolic blood pressure in the 10 patients who were treated with taurine decreased by 9.0 ± 2.9 mm Hg (mean ± SE; p > .05 by paired t test), compared with a 2.7 + 2.3 mm Hg decrease (NS) in the nine patients treated with placebo and diastolic blood pressure in the taurine-treated patients decreased by 4.1 + 1.7 mm Hg (p > .05) compared with 1.2 ± 3.0 mm Hg (NS) in the placebo-treated subjects. In the patients receiving taurine plasma epinephrine (E) decreased significantly, with a negligible decrease in plasma norepinephrine (NE). The effect of taurine on plasma catecholamines and the response of plasma E after the stimulation with glucagon was also studied in 12 borderline hypertensive and nine agematched normotensive subjects. Basal plasma E was significantly higher in borderline hypertensive than in normal subjects, but basal plasma NE did not differ in the two groups. The intravenous bolus injection of glucagon (1 mg) caused a rapid and transient increase in plasma E in each subject studied; plasma E reached a peak concentration 2 to 5 min after the injection of glucagon and thereafter declined rapidly to the baseline level. The overall response of plasma E to glucagon stimulation was significantly (p > .01) greater in the borderline hypertensive than in the normotensive subjects. Taurine not only reduced mean blood pressure and basal plasma E, but also attenuated the increased response to glucagon in the borderline hypertensive subjects. In the normotensive subjects, however, the administration of taurine did not significantly change blood pressure, basal plasma catecholamines, or the response of plasma E to glucagon. Overall, there was a direct correlation (r = .670, p > .01) between the decrements in mean blood pressure and those in plasma E after taurine in the 22 borderline hypertensive subjects. Evidence presented suggests, therefore, that sympathoadrenal tone is increased in young borderline hypertensive individuals, and that oral administration of taurine attenuates increased tone, leading to the reduction of blood pressure.