Aerobic efficiency characterized by the amount of oxygen consumed, per kg of b.w. and per meter of distance while running at a steady pace of 12 km/h (in children 10 km/h) attained 0.18--0.21 in trained athletes over 16 years of age and 0.23--0.25 in children. Trained children had only marginally better efficiency than the untrained schoolchildren, and there was no significant difference among values found in the athletes, which can be explained by their experience in running. The larger was the fibre area in the thigh muscle of sprinters, the worse was the aerobic efficiency, while in long distance runners there was no correlation between these two parameters. Taking the error VO2 measurements into account, it is rather doubtful that "routine" determination of aerobic efficiency has any diagnostic value.