Plasma renin activity (PRA), dopamine beta-hydroxylase (DBH) activity, and plasma aldosterone concentration were determined in 51 patients categorized as borderline or persistent hypertensives and as high, normal, or low renin. Basal DBH activity correlated with diastolic blood pressure in borderline but not in persistent hypertensives. In borderline patients with normal or high renin activity standing plus furosemide caused a marked rise in diastolic blood pressure but little change in systolic and increased PRA and DBA. In contrast this stimulation procedure depressed both systolic and diastolic blood pressure and resulted in blunted PRA and DBH responses in the low renin borderline and the three groups of persistent hypertensives. After dietary sodium restriction, the decrease in systolic blood pressure correlated with the decrease in urinary sodium excretion in persistent, but not in the borderline patients. The data suggest that both borderline an persistent hypertensive patients respond to volume depletion with an increase in sympathetic and renin activity, and the increase of these two vasoconstrictive systems is greater in borderline hypertensives.