It has been suggested that the potential for training to alter the physiological responses to exercise in children is related to a “maturational threshold”. To address this, we investigated the interaction of swimtraining status and maturity on cardiovascular and metabolic responses to lower and upper body exercise. Twenty-one prepubertal [Pre: 11 trained (T), 10 untrained (UT)], 30 pubertal (Pub: 14 T, 16 UT), and 18 postpubertal (Post: 8 T, 10 UT) girls completed ramp incremental exercise on a cycle and an upper body ergometer. In addition to pulmonary gas exchange measurements, stroke volume and cardiac output were estimated by thoracic bioelectrical impedance, and muscle oxygenation status was assessed using near-infrared spectroscopy. All T girls had a higher peak O2 uptake during cycle (Pre: T 49 ± 5 vs. UT 40 ± 4; Pub: T 46 ± 5 vs. UT 36 ± 4; Post: T 48 ± 5 vs. UT 39 ± 8 ml˙kg-1˙min-1; all P < 0.05) and upper body exercise (Pre: T 37 ± 6 vs. UT 32 ± 5; Pub: T 36 ± 5 vs. UT 28 ± 5; Post: T 39 ± 3 vs. UT 28 ± 7ml˙kg-1˙min-1; allP < 0.05). T girls also had a higher peak cardiac output during both modalities, and this reached significance in Pub (cycle: T 21 ± 3 vs. UT 18 ± 3; upper body: T 20 ± 4 vs. UT 15 ± 4 l/min; all P < 0.05) and Post girls (cycle: T 21 ± 4 vs. UT 17 ± 2; upper body: T 22 ± 3 vs. UT 18 ± 2 l/min; all P < 0.05). None of the measured pulmonary, cardiovascular, or metabolic parameters interacted with maturity, and the magnitude of the difference between T and UT girls was similar, irrespective of maturity stage. These results challenge the notion that differences in training status in young people are only evident once a maturational threshold has been exceeded. © 2011 the American Physiological Society.