1. The present investigation was conducted to test the hypothesis that the respiratory system is stressed more during exhaustive exercise in chronic hypoxia than in normoxia. 2. Four healthy male subjects (aged 33-35 years) exercised on a cycle ergometer at 75% of the local maximum oxygen consumption (Vo2,max) until exhaustion, at sea level (SL) and after a 1 month stay at 5050 m (HA). 3. Airflow at the mouth (V), oesophageal (Po) and gastric (Pg) pressures were measured at rest, during exercise and recovery. Minute ventilation (VE), respiratory power (Wresp), respiratory frequency (f) and transdiaphragmatic pressure (Pdi) were calculated from the measured variables. 4. The subjects' mechanical power output of cycling at HA was 23.7% lower than at SL. In spite of this reduction, time to exhaustion at HA was 55.3% less than at SL. VE increased slightly during exercise at SL, but showed a marked increase at HA, and at the end of exercise at HA was 47.3% higher than at SL. 5. Respiratory power increased more at HA than at SL (77.3% higher at the end of exercise) due to the increase in f needed to sustain the high VE. 6. Gastric pressure swings were negative at the end of HA exercise but always positive at SL. The Pai:Po ratio reached values below 1 at HA but never at SL. 7. These data seem to indicate that the respiratory system is stressed more during submaximal exercise at HA than at SL. We suggest that the exceedingly high VE demand, requiring an excessive Wresp, may lead to fatigue of the diaphragm.