The sympathetic nervous system has been investigated in 42 children with family history of arterial hypertension and 68 children of the same age without hypertensive relatives. Pressure responses to mental arithmetic and to isometric handgrip stress were measured in both groups, along with 24 h catecholamine excretion. Resting blood pressure and heart rate did not show any difference between groups. The increase in diastolic pressure during the mental arithmetic exercise was, however, significantly greater in children with family history of hypertension. A sharp increase in blood pressure and heart rate was observed during isometric handgrip in both groups without any significant difference. Twenty-four hour catecholamine urinary excretion was significantly higher in the group with family history of hypertension (31 ± 15 vs. 23 ± 13 µg/24 h; p < 0.05). These results seem to indicate that an initial impairment of the sympathetic activity is already detectable in young offspring of hypertensive patients.