The present study addressed the effects of carbohydrate consumption during endurance exercise on performance, energy turnover, and metabolism. Well-trained endurance runners consumed a beverage with (cho[+]) or without (cho[-]) carbohydrates during a long-distance run (46.6 km). The respiratory quotient (RQ), plasma levels of carbohydrate and fat metabolites, and of hormones (insulin, glucagon) were measured before, several times during, and after the run. The mean running speed for the entire distance was 13.6 and 13.4 km/h with the cho[+] and cho[-] beverage, respectively. The decrease in speed that was observed towards the end of the run was somewhat more pronounced with consumption of the cho[-] beverage. The RQ decreased during the run almost linearly. This decrease was independent of the consumed beverage. The changes in plasma levels of lactate, free fatty acids (FFA), glycerol, D-3-hydroxybutyrate (DHB), glucagon and insulin that occurred during the run were not affected by intake of the cho[+] beverage. However, intake of the cho[+] beverage prevented the decrease in plasma glucose observed towards the end of the run under control conditions, and eliminated the steep postexercise increase in plasma DHB. The intake of the cho[+] beverage also caused a rapid decrease in plasma levels of FFA and glucagon after the run, and slightly increased plasma insulin. The results demonstrate that ingestion of a carbohydrate-containing beverage during a long-distance run affects metabolism only during the final phase of the run and during the subsequent recovery period. Moreover, carbohydrate consumption improves performance only during the final phase of a long-distance run.