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The purpose of this study was to examine the relationship between urinary norepinephrine excretion and blood pressure, and to determine the influence of race on this relationship. Urinary norepinephrine was used to estimate renal and systemic sympathetic nervous system (SNS) activity.Fifty black and forty-nine white normotensive children aged 9–14 years had their blood pressure measured, and provided an overnight urine sample.Urinary norepinephrine was measured by radioenzymatic assay. Excretion rates of norepinephrine were expressed per milligram urinary creatinine.Black children had age-adjusted mean diastolic and systolic blood pressure which was higher than in white children. For both blacks and whites, nocturnal urinary norepinephrine excretion rates were positively related to age-adjusted mean diastolic blood pressure, but not to systolic blood pressure. Norepinephrine excretion was significantly lower in black children compared with white children.These findings suggest that the higher blood pressures in black children were not casually related to greater SNS activity. The SNS may have been suppressed in black children, possibly by a greater expansion of plasma volume; alternatively, black children may have been more sensitive to the influences of the SNS than white children.