Diaphragm fatigue may contribute to respiratory failure. ³¹P-nuclear magnetic resonance spectroscopy is a useful tool to assess energetic changes within the diaphragm during fatigue, as indicated by P_i accumulation and phosphocreatine (PCr) depletion. We hypothesized that loaded breathing during hypoxia would lead to diaphragm fatigue and inadequate aerobic metabolism. Seven piglets were anesthetized by using halothane inhalation. Diaphragmatic contractility was assessed by transdiaphragmatic pressure (Pdi) at end expiration with the airway occluded. A nuclear magnetic resonance surface coil placed under the right hemidiaphragm measured P_i and PCr during four conditions: control, inspiratory resistive breathing (IRB), IRB with hypoxia, and recovery (IRB without hypoxia). IRB alone resulted in hypercarbia (32 ± 7 to 61 ± 21 Torr) and respiratory acidosis but no change in diaphragm force output or aerobic metabolism. Combined IRB and hypoxia resulted in decreased force output (Pdi decreased by 40%; from 30 ± 17 to 19 ± 11 mmHg) and evidence of metabolic stress (ratio of P_i to PCr increased by 290%; from 0.19 ± 0.09 to 0.74 ± 0.27). We conclude that diaphragm fatigue associated with inadequate aerobic oxidative metabolism occurs in the setting of loaded breathing and hypoxia. Conversely, aerobic metabolism and force output of the diaphragm remain unchanged from control during loaded normoxic or hyperoxic breathing despite the onset of respiratory failure.