For the purpose to clarify whether physical training during the peak height velocity period of growth can accelerate the development of aerobic power and cardiorespiratory function, we determined the anaerobic threshold, VO₂ (AT) and maximaum O₂ uptake, VO₂ (MAX) together with cardiac output and several parameters indicating respiratory function in 16 boys. Stroke volume (SV), heart rate (HR) and cardiac output (Q) were measured continuously by using an automated impedance cardiograph. This apparatus can print out the above parameters once every 10 to 20 cardiac cycles in analog or digital form. An ensemble averaging technique was applied to improve the signal to noise ratio of the impedance signal during exercise. Minute ventialttion (V_E), O₂ uptake (VO₂), CO₂ output (VCO₂), end-tidal CO₂ tension (P_ETCO₂) and respiratory quotient were simultaneously measured breath by breath. The subjects were classified into two groups, the one trained group (T) and the other control group (C) . The boys included in T were 16 yrs and were engaged in the systematic training of Judo since 4 to 8 years ago. The boys of C were of the same age and were received no systematic physical training. The body weight of T was greater than C by 24%. The exercise was performed using a bicycle ergometer of which load was increased in step by 10 W every 30 sec. The measurement was terminated when VO₂ reached a plateau. AT was determined later on the record adopting the criteria proposed by Wasserman et al. i. e., the time point when V_E/VO₂ increased dominantly with constan V_E/VCO₂. At lest, SV, V_E and VO₂ were greater in T by 24, 20 and 36%, respectively. There was no significant difference in Q. At AT, the above parameter were greater in T than C by 36, 43 and 31%. At VO₂ (MAX), SV, Q, V_E, and VO₂ were larger in T by 56, 42, 34, and 27%. The difference between C and T was however disappeared when the parameters were normalized on body weight. There were significant correlations between body weight and either of VO₂ (AT) or VO₂ (MAX) . The same held true between the maximum value of SV and the above aerobic parameters. It was concluded that Judo training during the growth period enhances the muscle mass primarily, and the aerobic power is developed so as to match the increased mascle mass. The development of cardiac function is a major determinant for anaerobic threshold and maximum O₂ uptake.