Muscle metabolism during one‐legged submaximal exercise executed in a hypobaric chamber (corresponding to 2300 m above sea level) was studied by percutaneous muscle biopsies from the vastus lateralis and compared with corresponding data from the other leg, exercising at the same absolute load under normobaric conditions on another Occasion. After 15 min of exercise, the adenosine triphosphate/adenosine diphosphate ratio was lower and inosine 5‐monophosphate higher in the hypobaric than in the normobaric leg. The possibility is suggested that this metabolic alteration stimulates increased expression of oxidation‐related mitochondrial enzymes. Despite this sign of energy depletion, there was no indication of increased glycolysis. A tendency to increased citrate concentration in the hypobaric leg could suggest an increased mobilization and utilization of fat, which auld have counteracted any stimulating effect on the glycolytic