To investigate cardiorespiratory function during circumpubertal growth, 62 boys (aged 9–10 yr) were studied annually for 6 yr. Measurements of O2 uptake (VO2), cardiac output, and arteriovenous O2 difference were made during a submaximal bicycle test. Values were interpolated to a heart rate of 155 beats X min-1 (VO2 at a heart rate of 155) for comparisons across ages 10.8–14.8 yr. To account for growth differences among the boys, data were also aligned at yearly intervals relative to their individual age of peak height velocity (PHV; maturative age). The group was further divided into early, mid, and late maturers based on their year of PHV. VO2 and stroke volume (SV) of late maturers were larger at each maturative age. SV mirrored the increase in VO2 at all stages of development except during the period of most rapid growth. Arteriovenous O2 difference showed an increase in the year of peak growth with little change during any of the other maturative age intervals. Multiple regression analysis indicated that VO2 was determined primarily by the size of SV throughout this age range.