To evaluate sodium susceptibility in subjects with borderline hypertension at increased risk of developing essential hypertension, the effect of salt loading after sodium deprivation with a diuretic was studied in 21 young patients with borderline hypertension and 12 age-matched normal subjects. Treatment with a diuretic caused significant decreases in mean blood pressure (MBP) in subjects with borderline hypertension but not in normotensive subjects. In borderline hypertensives, the subsequent sodium loads resulted in a significant rise in MBP (5.8 +/- 1.7%; p less than .01), but sodium did not change MBP in normotensives. There is a good correlation between the increments in MBP with sodium loads and the decrements in MBP with a diuretic for each patient (r = -.759, p less than .001). After diuretics, cardiac index (CI) as measured echocardiographically fell significantly but calculated total peripheral resistance (TPR) remained unchanged in subjects with borderline hypertension. After 180 meq sodium chloride each day was added for 7 days, CI (9.1 +/- 2.1%; p less than .01) and stroke index (21.0 +/- 3.4%; p less than .01) rose significantly but TPR remained unchanged. Overall, the increments of MBP with sodium loads did not correlate with the changes in CI but did correlate with the changes in TPR (r = .567, p less than .01). In these young patients with borderline hypertension, plasma norepinephrine and epinephrine concentrations and plasma renin activity (PRA) were significantly higher than in normotensive subjects.(ABSTRACT TRUNCATED AT 250 WORDS)