This study compared the effects of moderately intense exercise in hot and thermoneutral environments on muscle glycogen and carbohydrate utilization. Well-trained, heat acclimatized cyclists (n = 7) rode at 73.6 +/- 1.1% maximal oxygen consumption for 60 min in a thermoneutral room (23.5 +/- 0.6 degrees C, 52.7 +/- 2.9 relative humidity) or an environmental chamber (33.7 +/- 0.1 degrees C, 49.1 +/- 1.8% relative humidity). During each exercise bout, the subjects received 125 ml of water every 15 min. Muscle biopsies from the vastus lateralis were obtained prior to and following each exercise bout. Exercise in the heat significantly elevated rectal temperature and heart rate above and reduced body weight and plasma volume below that produced by exercise in a thermoneutral environment. Plasma glucose and blood lactate concentrations were similar between treatments prior to exercise, but increased to a greater concentration (p < 0.05) when exercise was performed in the heat. No differences between treatments were found for blood glycerol or free fatty acid concentrations, carbohydrate oxidation or muscle glycogen utilization. These results suggest that moderately intense exercise in the heat, as opposed to a thermoneutral environment, does not increase the rate of muscle glycogenolysis or carbohydrate oxidation in well conditioned, heat acclimatized subjects.