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Oxygen free radicals may play roles in hypertension both in arteriolar constriction and in formation of lesions.To quantify free radical production in blood plasma of genetic hypertensives.Hydrogen peroxide levels were measured, because it is one of the most stable reactive oxygen species.An electrode technique was used to determine plasma hydrogen peroxide levels after blockade of endogenous catalase with sodium azide. This method was validated by an independent spectrophotometric technique.Members of the essential hypertensive group (n = 21) had higher plasma hydrogen peroxide levels (3.16 ± 0.14 versus 2.50 ± 0.16 mmol/l, P = 0.005) than did members of the normotensive group (n = 29). Furthermore, within the normotensive group, those with a family history of hypertension (n = 15) exhibited higher hydrogen peroxide levels (2.83 ± 0.27 versus 2.14 ± 0.13 μmol/l, P = 0.03) than did those without such a family history (n = 14). Plasma hydrogen peroxide levels in these 50 subjects were correlated to their mean arterial pressures (r = 0.54, P < 0.001). When hypertensives were grouped with normotensives without a family history of hypertension, the correlation improved (r = 0.70, P < 0.001). Statistical analysis (two-way analysis of variance) revealed that a family history of hypertension was a better predictor of plasma hydrogen peroxide production than was blood pressure status (P = 0.003 versus P = 0.093). Further investigations showed that superoxide is produced in plasma and that one of its sources is xanthine oxidase.Hydrogen peroxide is produced in blood plasma and elevation of its level could constitute a pathogenic factor in vascular organ damage attendant upon systemic hypertension. J Hypertens 16:291-303 (c) 1998 Rapid Science Ltd.