Aerobic capacity ((V)O2 max) after treadmill running was determined for 60 girls of 10 to 18 years of age. (V)O2 max increased from an average value of 861 ml/min in girls of 10 years to 1484 ml/min in the 17-18 years group. (V)O2 max per kg body weight was, however, reduced from an average value of 40 ml/min in pre-pubertal age groups to around 36 ml/min in post-pubertal girls. Maximum pulmonary ventilation ((V)E(max)) increased from 36 L/min in 10 years' girls to 54 L/min in the 17-18 years group. Corresponding increase in tidal volume (VT(max)) was from 527 ml to 1043 ml, respiratory rate being reduced from 69 to 53 per min. Respiratory equivalent, however, did not show any significant difference and the average value was 40 L of air per L of O2. On an average (V)E(max)) was 65% of the maximum voluntary ventilation and VT(max) was 45% of the vital capacity. These two ratios also did not differ with age. The maximum pulse rate as well as the recovery pulse rate response up to 3 min of recovery did not show any significant change with age. (V)O2(max) and (V)E(max) were found to be proportional to height2.2 and height 1.7 1.7 respectively and these were close to the expected value of height2. It appears that growth in aerobic capacity and its related functions in the girls are proportional to their growth of linear dimension and the reduction of (V)O2(max) per kg body weight in the post-pubertal age group may largely be attributed to the increase in the body fat after the puberty.