The purpose of this investigation was to assess the cardiorespiratory functions during endurance running in young children. Eighteen pre-school boys and girls (mean 6.32 years old) participated in this study as subjects. With progressive ground running method, peak oxygen intake was determined by the Douglas bag method. In addition, heart rate and running speed during 750m and 1500m runs were measured. The results obtained in this investigation were as follows: Peak oxygen intake for boys was 38.3±5.76ml/kg・min and for girls 36.7±3.84ml/kg・min. The linear relationship between running speed and oxygen intake, and between heart rate and oxygen intake was obtained during exercise. Therefore, estimation of the intensity of endurance running was considered to be possible through the measurement of heart rate. At the onset of 750 and 1500m runs, heart rate was increased rapidly and was attained to steady state level in about 40 seconds. In this level heart rate indicated about 190-200 beats/min, which was comparable to the maximal heart rate of subjects. The intensity of the endurance running, therefore, was estimated to be approximately maximum.