Age, height, mass, fat-free mass and vital capacity were used as predictors of maximum aerobic power (VO2 max). The variables were cast in linear form by logarithmic transfomation and submitted to multiple regression analysis. Results indicate VO2 max as a power function of age, height and mass in 50 untrained boys aged 7 to 13 years. In this group the relationship between VO2 max and body mass may be expressed by the equation Y=0.076X0.88 (r=0.92, P <0.01). Age, height and mass together accounted for 89 per cent of the variance in VO2 max (R=0.94, P <0.01). In 30 girl swimmers and in 14 young boys during 22 months of running training, VO2 max was proportional to body mass and indicated greater maximum aerobic power for their size and age. In normally growing children, VO2 max appears to increase more slowly than body mass. Children subjected to aerobic training evidently maintain VO2 max in proportion to their increasing mass throughout adolescence.