SummaryAlthough body size and muscle mass increase considerably during growth in children, certain aerobic responses to exercise appear to be regulated so that the delivery of oxygen to muscle is maintained at optimized levels. We proposed that the relationship between oxygen uptake, (&OV0312;O2) and heart rate (HR) was one of the regulated responses. We further hypothesized that the increase in &OV0312;O2per increase in HR during progressive exercise would differ in subjects of different size, but when normalized to body weight would be constant since changes in muscle mass are highly correlated to changes in body mass. To test this, we performed a cross-sectional study of 107 normal children, 50 girls and 57 boys ranging in age from 6 to 17 years. The protocol consisted of a continuously increasing work rate on a cycle ergometer, to the limit of the child's tolerance (ramp forcing function). Gas exchange was measured breath-by-breath for the determination of &OV0312;O2, and heart rate was measured beat-by-beat. We used linear regression techniques to determineM, the slope, andB, theyintercept of the equation: &OV0312;O2=Mx HR -B. In both boys and girls,Mincreased significantly with body weight, but when normalized for body weight (M/kg), there was no systematic change with increasing weight or age, the mean value being 0.33 ± 0.10 ml/min/kg (SD). The mean value for the boys was 0.37 ± 0.10 which was significantly greater than that of the girls (0.29 ± 0.08,p<0.01). Using allometric equations, we foundM,B, and the O2-pulse (&OV0312;O2/HR) at a heart rate of 140 beats/min and at the anaerobic threshold, all scaled in direct proportion to body weight (i.e., to 1.0 power of body weight). We conclude that during growth, the output of the heart is closely tied to the size of the muscles so that delivery of oxygen during exercise is maintained at optimized levels from early in childhood.