The effect of endurance training on aerobic power and the relationship between aerobic power and running performance were investigated longi tudinally on eleven junior runners for 5-7 years, starting from the age of 14, 15 yr. Maximal aerobic power was measured with the subject running on a motor-driven treadmill. The subjects were divided into two groups (Group I, II) .
The results were as follows ;
1) Group I (superior junior runners) showed greater aerobic power and better running performance compared to Group II (good junior runners) . The values for Group II were between those for Group I and for ordinary school boys.
2) Aerobic power for Group I increased from 14, 15 to 18, 19 years old, while aerobic power for Group II decreased to the similar values of the ordinary school boys after stopping the running training.
3) Greatest value for aerobic power was obtained on Subject A as follows : 3.63 1/min (61.5 ml/kg. min) at 14.7 yr of age, 4.67 1/min (74.6 ml/kg. min) at 17.8 yr, 5.04 1/min (76.3 ml/kg. min) at 20.7 yr.
4) Improvement in running performance was closely related to the increase in aerobic power (1/min) for the junior runners during the age of 14 to 18 yr.