The metabolic cost (VO2) of running was studied on a motor-driven treadmill in nine athletic boys, five athletic girls, and nine active boys aged 11--13 years and the results compared with their performance times during racing out of doors. On 15 of the children, additional observations of the effects of external loading on aerobic power output were made. The results showed that VO2 was proportional to body weight in children but when expressed in ml . kg-1 . min-1, VO2 for a given speed of running was significantly higher in children than expected from previously collected data on adults. There were no significant differences between aerobic cost of running of the athletic boys, girls, or the active boys. The increased VO2 ml . kg-1 . min-1 in children appeared to be independent of stride length and frequency but external loading equivalent to 5% of body weight reduced VO2 (ml . kg-1 . min-1), particularly at the higher speeds. It was suggested in young active and athletic children due to their relatively light body weights and highly developed aerobic power outputs, that the required frequency of leg movement was not optimally matched to the force necessary to produce the most economic conversion of aerobic energy into mechanical work. Thus, in competitive events their performance times were related to their maximal aerobic power output (r = -0.75) but their times were always inferior to those which one might have expected from previous aerobic power weight data collected on adult male and female athletes.