Relationships between renal haemodynamics and the renin-angiotensin system were examined in 128 male adolescents with differing predispositions to hypertension. Baseline renal vascular resistance (RVR) was significantly elevated in the normotensive subjects with a positive family history, NT (FH+) group, than in the normotensives with a negative family history, NT (FH-) group, while there were no significant differences in other characteristics between the two groups. The borderline hypertensive subjects with a positive family history, BH (FH+) group, showed an even bigger increase in RVR and also had elevated plasma renin activity (PRA). In the NT (FH+) and BH (FH+) groups RVR correlated significantly with PRA levels. When captopril was given, the increased RVR in the NT (FH+) and BH (FH+) groups decreased, in correlation with baseline PRA, to levels not significantly different from those in the NT (FH-) group, in which no significant changes occurred in RVR. These results suggest that RVR may increase primarily in adolescents genetically predisposed to essential hypertension and that the renin-angiotensin system may play an important role in the mechanism of this increase in RVR.