The blood pressure and plasma norepinephrine response to oral tyrosine, the precursor of norepinephrine, supplementation (2.5 g t.i.d.) of regular meals was examined in 13 untreated patients with mild essential hypertension. Using a randomized double-blind crossover design, each 2-week treatment was followed by a 2-week supplement-free interval. Supine and standing blood pressure and plasma norepinephrine levels were measured at the beginning and end of each 2-week treatment. Plasma tyrosine levels increased (p less than 0.001) from 71.2 +/- 8.0 nM/ml at baseline to 152.8 +/- 17.4 nM/ml 2 hours after the tyrosine supplement. Blood pressure under control conditions was 144 +/- 3 Hg systolic, 91 +/- 2 mm Hg diastolic (109 +/- 2 mm Hg mean) after 30 minutes in the supine position and 148 +/- 4 mm Hg systolic, 102 +/- 3 mm Hg diastolic (117 +/- 3 mm Hg mean) after 5 minutes of standing. Plasma norepinephrine levels were 191 +/- 18 pg/ml in the supine subjects and 390 +/- 33 pg/ml in the standing subjects. No difference in systolic, diastolic, or mean blood pressure, heart rate, or plasma norepinephrine levels were seen between the beginning and end of each period or between groups. Individual changes in blood pressure showed no correlation with individual changes in norepinephrine levels. These results indicate that the addition of a tyrosine supplement to the usual diet of mild hypertensive subjects has no beneficial effect on blood pressure.