We studied the effect of high and low NaCl diets in normotensive and borderline hypertensive subjects to determine if a high NaCl diet produces abnormal renal vasoconstriction during the stress of upright posture in borderline hypertensive subjects. We studied 13 normotensive young men with diastolic blood pressures below 85 mm Hg and nine borderline hypertensive young men defined by diastolic blood pressures intermittently above 90 mm Hg. The subjects achieved comparable sodium balance during 6 days of low NaCl (10 mEq Na, 40 mEq Cl, 100 mEq K) and high NaCl (400 mEq Na, 400 mEq Cl, 100 mEq K) diets. In the normotensive subjects, standing for 30 minutes resulted in a tendency for diastolic blood pressure to fall during both diets. In contrast, during standing borderline hypertensive subjects showed no change in diastolic blood pressure during the low salt diet and a tendency for diastolic blood pressure to increase after the high salt diet. Standing reduced renal plasma flow in both groups during both diets. However, only during the high NaCl diet did the absolute decrease and percent decrease in renal plasma flow during standing differ significantly (p less than 0.05 and p less than 0.01, respectively) between the borderline hypertensive (-151 +/- 24 ml/min/1.73m2; -29 +/- 4%) and normotensive subjects (-79 +/- 17 ml/min/1.73m2; -15 +/- 3%). The resultant increase in the renal vascular resistance index with standing did not differ between the two groups during the low NaCl diet.(ABSTRACT TRUNCATED AT 250 WORDS)