The first purpose of this study was to investigate whether a glucose (GLU)+fructose (FRUC) beverage would result in a higher exogenous carbohydrate (CHO) oxidation rate and a higher fluid availability during exercise in the heat compared with an isoenergetic GLU beverage. A second aim of the study was to examine whether ingestion of GLU at a rate of 1.5 g/min during exercise in the heat would lead to a reduced muscle glycogen oxidation rate compared with ingestion of water (WAT). Eight trained male cyclists (maximal oxygen uptake: 64+/-1 ml.kg-1.min-1) cycled on three different occasions for 120 min at 50% maximum power output at an ambient temperature of 31.9+/-0.1 degrees C. Subjects received, in random order, a solution providing either 1.5 g/min of GLU, 1.0 g/min of GLU+0.5 g/min of FRUC, or WAT. Exogenous CHO oxidation during the last hour of exercise was approximately 36% higher (P<0.05) in GLU+FRUC compared with GLU, and peak oxidation rates were 1.14+/-0.05 and 0.77+/-0.08 g/min, respectively. Endogenous CHO oxidation was significantly lower (P<0.05) in GLU+FRUC compared with WAT. Muscle glycogen oxidation was not different after ingestion of GLU or WAT. Plasma deuterium enrichments were significantly higher (P<0.05) in WAT and GLU+FRUC compared with GLU. Furthermore, at 60 and 75 min of exercise, plasma deuterium enrichments were higher (P<0.05) in WAT compared with GLU+FRUC. Ingestion of GLU+FRUC during exercise in the heat resulted in higher exogenous CHO oxidation rates and fluid availability compared with ingestion of GLU and reduced endogenous CHO oxidation compared with ingestion of WAT.