The purpose of this study was to measure the oxidation of ethanol at rest and during prolonged moderate exercise with use of 13C labeling. Five healthy young males (22.4 +/- 2.7 yr; maximal O2 uptake = 56 +/- 6.6 ml.kg-1.min-1) performed three exercises (68.4 +/- 6.7% maximal O2 uptake; 90 min) on a cycle ergometer with ingestion of 0.4 (trial A) and 0.8 (trial B) g/kg body wt of [13C]ethanol (diluted in 770 +/- 72 ml of water) or water only (trial C). The subjects were also studied during a 90-min rest period after the ingestion of 0.8 g/kg body wt of [13C]ethanol (trial D). At rest, over the 90-min observation period, only 2.1 +/- 0.3 g of the 61.6 +/- 5.7 g of ethanol ingested were oxidized, providing 11.1 +/- 1.9% of the total energy expenditure. Over the 90 min of exercise, the amounts of ethanol oxidized were similar in trials A (9.5 +/- 2.0 g) and B (8.5 +/- 2.5 g). The contribution of ethanol represented 5.2 +/- 1.0% of the total energy expenditure, which is much lower than that previously reported for exogenous carbohydrates (8-18%) or medium-chain free fatty acids (7-14%). The small contribution of ethanol to energy metabolism did not significantly modify endogenous substrate oxidation.