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Office, i.e. measured by the physician at rest, and 24h ambulatory systolic (SBP) and diastolic (DBP) blood pressures, heart rate, supine and upright plasma renin activities, supine and upright aldosterone concentrations and plasma and urine sodium and potassium were measured in 61 young male subjects aged 19–25 years, including 40 normotensive subjects (office DBP ≤90mmHg and office SBP ≤140mmHg) and 21 borderline hypertensive subjects (non-normal blood pressures with office DBP ≤95mmHg and office SBP ≤160mmHg). No significant differences were found in the plasma or urine K+ or Na+, upright or supine plasma renin activity or aldosterone concentration between normotensives and borderline hypertensive subjects. No correlation was detected between plasma and urine K+ or Na+, upright and supine aldosterone concentration or supine plasma renin activity and blood pressure. In contrast, significant inverse correlations were observed between upright plasma renin activity and blood pressure. The correlations were approaching statistical significance when upright plasma renin activity was related to office SBP and office DBP (r=-0.22, P=0.097 and r=- 0.25, P=0.049, respectively), and were more significant when plasma renin activity was related to 24h mean DBP (r=-0.32, P=0.013) and to SBP and DBP standard deviations (r=-0.37, P=0.004 and r=-0.26, P=0.04, respectively). The correlations were highly significant when upright plasma renin activity was related to the SBP and DBP day–night fluctuation, the latter being calculated by the cosinor model (r=-0.37, P=0.003 and r=-0.43, P=0.001, respectively). The results suggest that plasma renin activity may already be involved in borderline essential hypertension.