M-mode echocardiograms were obtained on 651 healthy subjects, 7-22 years of age, whose diastolic blood pressure levels remained in the same height-, race-, and sex-specific decile during two biannual examinations. Echocardiographic measures of heart size and dynamics were compared across the total blood pressure distribution. Left ventricular stroke volume, cardiac output and ejection fraction, minor axis shortening, velocity of circumferential fiber shortening, and peripheral vascular resistance were correlated with blood pressure levels. There were positive correlations (p less than .001) of cardiac output and stroke volume with both systolic and diastolic blood pressure levels. Left ventricular output and stroke volume were associated with measures of body size, especially height, weight, ponderal index, and body surface area (p less than .001). The left ventricular output and stroke volume increased with age and with systolic blood pressure quintiles in the four race-sex groups. With adjustment for systolic blood pressure and measures of body size, white males had greater cardiac output (1.25 l/minute for ages 18-22 years, p = .01) and stroke volume than black males. Black males had higher peripheral resistance (4.5 mm Hg/(l/minute), p = .01) than whites. These results suggest that different hemodynamic mechanisms operate in the early phase of hypertension in blacks vs. whites in this population.