Chronic exposure to high altitude is known to result in changes in the mechanisms regulating O₂ delivery to the contracting muscle. However, the effects of acclimatization on metabolism in the contracting muscle cell remain unclear. In this study, we have investigated the hypothesis that acclimatization would result in a closer coupling between ATP utilization and ATP production and that the improved energy state would be accompanied by a reorganization of the metabolic pathways consisting of an increased oxidative and decreased glycolytic potential. Five men, mean age of 28 ± 2 (SE) yr, performed a standardized, two-stage submaximal cycling task in normoxia for 20 min at each of 59 and 74% peak O₂ consumption before and 3-4 days after returning from a 21-day expedition to Mount Denali (6,194 m). Acclimatization was without effect in altering the resting values of the adenine nucleotides (ATP, ADP, AMP), inosine monophosphate (IMP), or phosphocreatine (PCr) in the vastus lateralis. During exercise (40 min) after acclimatization compared with preacclimatization, PCr was not as depressed (33.2 ± 7.1 vs. 40.6 ± 5.4 mmol/kg dry wt) and IMP (0.289 ± 0.11 vs. 0.131 ± 0.03 mmol/kg dry wt) and lactate (26.1 ± 6.2 vs. 18.6 ± 8.8 mmol/kg dry wt) in contracting muscle were not as elevated (P < 0.05). Although no effect of acclimatization was observed for the maximal activity (mol · kg protein¹ · h¹) of citrate synthase (4.76 ± 0.44 vs. 4.94 ± 0.45), lactate dehydrogenase was increased by 13% (36.5 ± 2.6 vs. 41.2 ± 3.1, P < 0.05). It is concluded that acclimatization results in an improved energy state in the contracting muscle when tested under normoxic conditions; however, these effects are not associated with a higher oxidative potential or a lower glycolytic potential as hypothesized.