Aerobic work capacities of rural children and adolescents, aged from 4 to 18 years, were determined. The whole number of subjects were 679 (368 boys and 311 girls respectively). Maximal oxygen uptake (VO_2 max.) and the performance of 5 minute run were measured by means of track run. The fol1owing results were obtained; 1) Absolute values of VO_2 max. increased almost linearly from 0.7921/min at 4 years of age to 2.062 l/min at 13 years of age for the boys. The corresponding increment for girls was from 0.7661/min to 1.982 1/min. During the course of such years boys were significantly superior to girls up to 9 years of age. No significant differences were found for the ages from l0 to 13 years. After 13 years of age boys showed sharp increase up to age 15 years to attain 3.0 l/min and then reached plateau. On the other hand girls showed only slight increase, that is, they exhibited the state of plateau after 13 years of age. 2) At all the ages relative values of VO_2 max. in terms of body weight were significantly greater for boys. This confirms that sex differences in aerobic work capacity already exist even in small children. 3) The performance of endurance run is expected to relate thoretically with the relative value of VO_2 max. in terms of body weight. The observed performance of 5 minute run, in fact. was significantly higher for the boys at all the ages. The performance of the boys gradually increased up to age 15 years (the distance covered was 1343 m), Where the relative value of VO_2 max. also reached the highest peak of 58.3 ml/kg/min, and dfter 15 years of age a state of plateau appeared. For the girls the distance covered in 5 minutes run gradually increased and reached the value of 1138m, where the relative value of VO_2 max. showed the second peak of 48.4 ml/kg/min and after that, quite different from the boys, the performance gradually decreased with advance in age. 4) According to the formula of allometry ; y=b.x^a, the three phases were found in the relationship between body height (h ; cm) and the absolute value of VO_2 max. (y ; l/min) for both sexes. The formulae, intervening the two critical points, were expressed as follows; for boys y = 3.0444×10^-5×h^2.2003 (132.8 cm,1.225 l/min) y=4.7022×10^-7×h^3.0658 (167.4 cm, 3.088 l/min) y =1.4935×10^2×h^1.0413 for girls y = 1.0700×10^-4×h^1.9250 (117.4 cm, l.026 l/min) y=5.0316×10^-7×h^3.0369 (149.8 cm, 2. 033 l/min) y =2.5276×10^-3×h^1.3333 The values ofα in each corresponding phase were almost the same in the both sexes. 5) In the relationship between body height(h ; cm) and the 5 minute run (y ; m) the two phases were found also for both sexes. For boys y=4.8867×h^1.1052 (155.4 cm, 1,296 m) y=2.3508×10^2×h^0.3384 for girls y = 4.1191×h^1.1194 (144.7 cm, 1,080 m) y=1.0832×10×h^0.0007 The values of α in each corresponding phase were also almost the same in the both sexes.