Renal function in the basal state and after sodium load has been investigated in 21 borderline, hypertensive, first-degree relatives of established hypertensives and in 21 age-and sex-matched, normal subjects with no family history of hypertension. During intravenous infusion of inulin and /?-aminohippurate in saline, both groups showed a decrease in plasma aldosterone levels (p < 0.05) but renal plasma flow (595 ± 48 vs. 750 ± 59 ml/min, p < 0.05), diuresis (1.4 ± 0.2 vs. 2.2 ± 0.5 ml/min, p < 0.05), natriuresis (0.21 ± 0.02 vs. 0.3 ± 0.02 mEq/min, p < 0.05) and sodium clearance (1.05 ± 0.1 vs. 2.4 ± 0.4 ml/min, p < 0.05) in borderline hypertensives were higher than in the control group. After the salt load (NaCl, 1.35% i.v., 5 ml/min for 2 h) there was an increase in blood pressure and a decrease in plasma aldosterone and potassium levels in both groups. However, borderline hypertensives showed higher diuresis, natriuresis, sodium clearance and also kaliuresis compared to normotensives. These results suggest that borderline hypertensives already present the changes in renal function which are characteristics of established hypertensives.