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Early differences in hemodynamic variables and vasoactive substances between progeny of hypertensive and normotensive parents were sought in normotensive children aged 10-17 years. Forty-two black and 34 white children of hypertensive parents (cases) and an age-balanced group of 20 black and 45 white children of normotensive parents (controls) underwent exercise stress testing. Blood pressure, heart rate, urinary electrolytes, kallikrein, and prostaglandin E-like material, plasma renin activity, and norepinephrine were measured before, during, and after exercise. Analyses compared the findings of four subject groups: black cases with black controls, white cases with white controls, black cases with white cases, black controls with white controls. Based on a family history of hypertension, significant familial differences were found: Black cases "were larger" (p = 0.003) when compared to black control subjects and had higher resting systolic, dlastolic, and maximum exercise diastolic blood pressures, and higher postexercise plasma renin activity (p = 0.04); white cases also were larger (p = 0.004) in comparison with white controls and had higher postexercise heart rates, higher preexercise urinary prostaglandin E-like material, and lower plasma norepinephrine after exercise (p < 0.05). Significant racial differences were noted in heart rate, blood pressure, urinary electrolytes, and other biochemical substances. Among the cases, black/white comparisons showed that blacks had lower postexercise heart rates, and higher resting and maximum exercise systolic and diastolic blood pressures (p < 0.04 for these comparisons). Additionally, blacks had higher preexercise urinary sodium excretion, lower preexercise urinary potassium excretion (p < 0.003), and lower plasma renin activity before and after exercise (p < 0.003). Finally, urinary kallikrein excretion before and after exercise was lower in blacks (p < 0.03). While blood pressures were similar among the black and the white controls, comparisons revealed that the blacks had higher preexercise sodium and potassium excretion (p < 0.005), lower plasma renin activity at all levels of activity, and lower plasma norepinephrine 10 minutes after exercise (p < 0.05). Thus, significant intraracial and interracial differences in physiological and biochemical variables, thought to be related to the development of hypertensive diseases, have been detected in normotensive children categorized according to a family history of hypertension. With use of these variables, the black case children can be separated from the other three groups and may have increased risk for the development of adult onset hypertension. (Hypertension 5: 56-70, 1983) KEY WORDS children essential hypertension