Six well-trained men (peak pulmonary oxygen uptake = 5.03 +/- 0.11 l/min) were studied during 2 h of exercise at 69 +/- 1% peak pulmonary oxygen uptake to examine the effect of carbohydrate (CHO) ingestion on glucose kinetics. Subjects ingested 250 ml of either a 10% glucose solution containing 6-[3H]glucose (CHO) or a sweet placebo every 15 min during exercise. Glucose kinetics were assessed by 6,6-[2H]glucose infusion corrected for gut-derived glucose in CHO. Plasma glucose was higher (P < 0.05) in CHO from 20 min. Total glucose appearance was higher in CHO due to glucose delivery from the gut (68 +/- 7 g), since hepatic glucose production was reduced by 51% (29 +/- 5 vs. 59 +/- 5 g). Glucose uptake was higher in CHO (96 +/- 7 vs. 60 +/- 6 g) with the ingested glucose supplying 67 +/- 4 g and, with the assumption that it was fully oxidized, accounted for 14 +/- 1% of total energy expenditure. In conclusion, CHO ingestion during prolonged exercise results in suppression of hepatic glucose production and increased glucose uptake. These effects appear to be mediated mainly by increased plasma glucose and insulin levels.