Cardiac function and dynamics were studied in 29 apparently normal children under 20 years of age. Table I lists the values obtained fro intracardiac and great vessel pressures. There was no correlation of these pressures with either age or increasing body size. Except for the possibility that our data is inadequate for patients under one week of age, these pressures as well as the ratio between pulmonary artery mean to systemic artery mean, appear to be constant throughout the pediatric age range. Arterio-venous differences ranged from 28 to 78 ml of O2/l of blood. The mean was 44.3 ml/l with anS.D. of 15.0. Left ventricular cavity diameters are shown for 11 normal children from this series and 9 patients from the literature. The 2 measurements recorded in the youngest subjects are elevated, probably as a result of our inability to distinguish that area of left ventricular ''wall'' occupied by the thymus. The ratio of the left ventricular cavity diameter to wall thickness was 7.0. Calculated systemic, total pulmonary and pulmonary arteriolar resistances appeared to be inversely related to age, height, and weight. Non-linear regression formulae employing the inverse of age, height and weight would be more suitable. Height alone gave the best correlation with all cardiac functions except output. Calculated resistance for both systemic and pulmonary circulations fall with increasing age at approximately the same rate. As systemic blood flow increases to accommodate the needs of increasing body size, the denominator of the DC resistance term increases and ''resistance'' decreases. Since in the absence of shunts, pulmonary blood flow must parallel the increase in systemic blood flow, pulmonary ''resistance'' decreases pari passu.
