There are conflicting data with regard to the effect of endurance training in children. On the basis of this information, the effects of 8 wk of run training on ventilatory threshold (VT) and VO2max of eight male children were investigated. Children ranged in age from 10 to 14 yr, with a mean age of 12.4 yr. All subjects were previously untrained. Training consisted of running 4 d.wk-1 for a period of 8 wk. Continuous running was performed 2 d.wk-1 for 10-30 min at 70-80% of VO2max. Interval running was performed the remaining 2 d.wk-1. Repeated intervals of 100-800 m at 90-100% of VO2max were used in this phase of the training. The total distance run for this type of training was 1.5-2.5 km. Incremental treadmill testing prior to and after the training period indicated a 19.4% increase in VT from 30.5 to 36.4 ml.kg-1.min-1 (P less than 0.05). When VT was expressed as a percentage of VO2max, there was a significant (P less than 0.05) increase from 66.6% to 73.8%. VO2max increased 7.5% from 45.9 to 49.4 ml.kg-2.min-2 (P less than 0.05). None of these changes was noted in eight age- and size-matched children who served as control subjects. The results of this study indicate that 8 wk of endurance running training which is of sufficient frequency, intensity, and duration can significantly improve VT and aerobic capacity in male children.