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Venous plasma noradrenaline (PNA) was recorded during a multistage exercise test in order to assess the possible role of excess sympathetic nervous system activity in young men at increased risk of developing essential hypertension, and to analyse whether any such involvement was associated with heredity and/or borderline hypertension. Four groups were evaluated: 28 normotensive (NTO) and 20 borderline hypertensive (BHO) offspring of hypertensives, 12 borderline hypertensives with normotensive parents (BH) and 28 normotensives with normotensive parents (NT). Analysis of variance showed that heredity per se was associated with an increased PNA response to light exercise. In NT, PNA correlated with age at rest and light exercise (r= 0.39-0.58, P<0.05-0.005). When the influence of age was controlled for by dividing the subjects into two age groups a more pronounced increase in PNA response to light exercise was found in the youngest age group of offspring hypertensives (17-26 years), whereas no differences between the groups were observed in the upper age group (27-40 years). Our results indicate the presence of a hyperreactivity of the sympathetic nervous system during light exercise in young offspring of hypertensives.