Litters of rats were exposed to normobaric normoxia or hypobaric hypoxia (P
= 450 mmHg) for 7 days at 3 different time points during early development (postnatal day (P)1, P6 & P11). A separate litter exposed to hypoxia at P11 was treated with the antioxidant Tempol (100 mg/kg) given by oral administration daily starting at P8. At P19, sternohyoid and diaphragm muscles were removed and isolated muscle bundles were mounted isometrically in physiological salt solution at 30°C in vitro. Fatigue was assessed in response to repeated stimulation (40 Hz) every 2 s for 5 min. Fatigue index was measured. Chronic hypoxia decreased sternohyoid, but not diaphragm, muscle endurance. Tempol treatment did not prevent hypoxia-induced muscle plasticity, suggesting that reactive oxygen species are not implicated in hypoxia-induced muscle dysfunction.