We hypothesize that after a prolonged period of acclimatization, i.e. 9 weeks, the reliance on lactate as a mean of distribution energy is higher compared to sea level due to a higher lactate rate of appearance and disposal from active skeletal muscle. To evaluate this hypothesis, 6 Danish lowlanders (25 ±2 years) were studied at rest and during 20 min of bicycle exercise at 146 watts at sea level (Sea Level) and after 9 weeks of acclimatization to 5260 m (Altitude). Whole body glucose rate of appearance (R_a) was similar at Sea Level and Altitude, both at rest and during exercise. Lactate R_a was also similar for those two conditions at rest; however, during exercise, lactate R_a was substantially lower at Sea Level (65 µmol.min^-1.kg^-1bw) than it was at Altitude (150 µmol.min^-1.kg^-1bw) at the same exercise intensity. During exercise net lactate release was about 6-fold at Altitude compared to Sea Level and, related to this, the tracer-calculated leg lactate uptake and release were both 3- or 4-fold higher at Altitude compared to Sea Level. The contribution of the two legs to glucose disposal (R_d) was similar at Sea Level and Altitude; however, the contribution of the two legs to lactate R_a was significantly lower both at rest and during exercise at Sea level (27 and 81%) than it was at Altitude (45 and 123%). In conclusion, at rest and during bicycle exercise, at the same absolute workload, carbohydrate and blood glucose utilization were similar at sea level and at 5260 m after 9 weeks of acclimatization. The leg net lactate release was several fold higher, and the contribution of leg lactate release to whole body lactate R_a was higher at 5260 m after 9 weeks of acclimatization compared to Sea Level. During exercise the relative contribution of lactate oxidation to whole body carbohydrate oxidation was substantially higher at 5260 m after 9 weeks of acclimatization compared to sea level, as a result of an increased uptake and subsequent oxidation of lactate by the active skeletal muscles.