The purpose of this study was to determine if differences in oxygen uptake kinetics and oxygen deficit existed between normal weight and severely overweight adolescent girls. Subjects included 10 normal weight and 8 severely overweight girls. The participants performed a leg cycling VO₂ peak test and a constant load leg cycling test at 80% of the ventilatory threshold (T-vent). In the constant workload test O₂ kinetics as indicated by Phase I (VO₂ L at 20 sec) and Phase II time constants (t) were determined. Also, the O₂ deficit (VO₂ L) was measured. As expected significant differences were noted in body composition and VO₂ peak relative to mass with normal weight body mass averaging 55.3 ± 7.0 kg, severely overweight 90.5 ± 18.0 kg, % fat normal weight 27.3 ± 3.9%, severely overweight 49.7 ± 4.9% and VO₂ peak (ml·kg^-1·min^-1) normal weight 32.0 ± 2.7 and severely overweight 22.0 ± 5.3. VO₂ peak (l·min^-1) and T-vent (%VO₂ max) were similar between groups. Results revealed similar O₂ kinetic responses between groups; phase I kinetics normal weight 0.72 ± 0.15 L; severely overweight 0.75 ± 0.13L, phase II (t) normal weight 41.5 ± 21.3 sec; severely overweight 33.9 ± 22.7 sec. However, the O₂ deficit was significantly higher in the severely overweight (0.75 ± 0.15L) when compared to the normal weight group (0.34 ± 0.13L). Correlations ranged from r = -0.15 to 0.51 between VO₂ peak (L·min^-1) or fat weight and phase I, t and O₂ deficit. These data generally support previous research concerning the independence of O₂ uptake response and body size.

• VO (L·min ) similar between the severely overweight and normal weight female youth

• Phase I and II O kinetic responses similar between severely overweight and normal weight female youth

• O deficit was significantly greater in the severely overweight participants