Chronic hypoxia occurs in normal individuals at altitude and also in patients suffering from respiratory disease. Skeletal muscle structure and oxidative capacity are age dependent are known to be affected by chronic hypoxia The aim of this study was to examine the age-dependent effects of chronic hypoxia on diaphragm muscle contractile and endurance properties. Adult (12 week old) and juvenile (3 week old) Wistar rats were exposed to either hypobaric hypoxia (barometric pressure 380mmHg) (n=12) or normobaric normoxia (n=12) for 6 weeks. At the end of the treatment periods, isometric contractile and endurance properties of isolated strips of diaphragm muscle were measured in tissue baths under hyperoxic (95%O2/5%CO2) conditions maintained at 30oC. Force-frequency relationship and fatigue tolerance was examined. For the force-frequency relationship, the diaphragm was stimulated at 10, 20, 30, 40, 60, 80 and 100 Hz (300ms train duration) allowing a 2-min interval between stimulation. Five minutes following the force-frequency protocol, fatigue was assessed in response to repeated tetanic contractions (40Hz, 300ms) every 2 seconds for 5 minutes. Specific force was reduced and fatigue resistance significantly increased in both adult and juvenile rats exposed to chronic hypoxia. The fatigue index (ie ratio of force at 5 min of fatigue to initial force) was significantly altered by chronic hypoxia in adult rats (32.5±4.8 vs. 51.7±3.2*; % of initial force, control vs. hypoxia*, P<0.05, ANOVA). This effect was similar in juvenile rats 43.0±1.9 vs. 67.4±3.3, % of initial force, control vs. hypoxia*, P<0.05, ANOVA). In this study, chronic hypoxia increased endurance in both adult and juvenile rats. This effect could be indicative of a fibre-type transition toward a more oxidative phenotype due to hypoxia or hypoxia-induced hyperventilation. Fatigue resistance was more pronounced in the juvenile compared to adult rats for chronic hypoxia groups. This study has shown developmental and hypoxia-induced plasticity in the rat respiratory muscle.