To assess mechanisms associated with the pressor effects of a high sodium diet in susceptible individuals, the hemodynamic and hormonal effects of sodium depletion and repletion were studied in 33 normal subjects and 30 subjects with borderline hypertension. The hypertensive group had significantly higher mean arterial pressure, weight, hematocrit, and upright plasma renin activity. Forearm hemodynamics were measured during periods of ad lib diet, 10 mEq, and 200 mEq sodium diet. The fall in forearm resistance during reactive hyperemia was inversely related to mean arterial pressure at rest (R = .400, p less than .005) and rose significantly in hypertensive subjects during salt depletion, 39 +/- 3.6 to 61 +/- 6.1 mmHg/ml/min/100 g (p less than .05). Sodium sensitivity in either normotensive or borderline hypertensive subjects was defined as an increase in mean blood pressure of more than 5% when sodium repleted. The individuals who were sodium sensitive had a higher forearm vascular resistance during sodium depletion than those who were sodium resistant, 67 +/- 10.5 versus 45 +/- 4.1 mm/ml/min/100 g (p less than .03). We conclude that young individuals with borderline hypertension already have alterations in vascular reactivity. This trait is shared by normotensive individuals whose blood pressure rises in response to sodium.