This study examined the effects of elevated muscle temperature on muscle metabolism during exercise. Seven active but untrained men completed two cycle ergometer trials for 2 min at a workload estimated to require 115% maximal oxygen uptake (VO2) either without pretreatment (CT) or after having their thigh wrapped in a heating blanket for 60 min before exercise (HT). HT increased (P < 0.01) muscle temperature (Tm) and resulted in a difference in Tm between the two trials before (delta = 1.9 +/- 0.1 degrees C, P < 0.01) and after exercise (delta = 0.6 +/- 0.2 degree C, P < 0.05). HT did not affect rectal temperature or plasma catecholamines. In addition, these parameters were not different between CT and HT either before or after exercise. No differences in resting intramuscular concentrations of the adenine nucleotides (ATP, ADP, AMP) or their degradation products (inosine 5'-monophosphate, ammonia), lactate, glycogen, creatine phosphate, or creatine were observed between HT and CT. During exercise, the magnitude of ATP degradation and inosine 5'-monophosphate and ammonia accumulation was higher (P < 0.05) in HT compared with CT. Although preexercise concentrations of glycogen and lactate were not different between the two trials, postexercise lactate concentration was higher (P < 0.05) and glycogen lower (P < 0.05) in HT compared with CT. In addition, net muscle glycogen use was higher (P < 0.05) in HT. It is concluded that an elevated Tm per se increases muscle glycogenolysis, glycolysis, and high-energy phosphate degradation during exercise. These alterations may be the result of an increased rate of ATP turnover associated with the exercise and/or changes in the anaerobic/aerobic contribution to ATP resynthesis.