To identify the mechanism underlying the exaggerated hyperglycemia during exercise in the beat, six trained men were studied during 40 min of cycling exercise at a workload requiring 65% peak pulmonary oxygen uptake (V̇(2peak)) on two occasions at least 1 wk apart. On one occasion, the ambient temperature was 20°C [control (Con)], whereas on the other, it was 40°C [high temperature (HT)]. Rates of glucose appearance and disappearance were measured by using a primed continuous infusion of [6,62H]glucose. No differences in oxygen uptake during exercise were observed between trials. After 40 min of exercise, heart rate, rectal temperature, respiratory exchange ratio, and plasma lactate were all higher in HT compared with Con (P < 0.05). Plasma glucose levels were similar at rest (Con, 4.54 ± 0.19 mmol/l; HT, 4.81 ± 0.19 mmol/l) but increased to a greater extent during exercise in HT (6.96 ± 0.16) compared with Con (5.45 ± 0.18; P < 0.05). This was the result of a higher glucose rate of appearance in HT during the last 30 rain of exercise. In contrast, the glucose rate of disappearance and metabolic clearance rate were not different at any time point during exercise. Plasma catecholamines were higher after 10 and 40 min of exercise in HT compared with Con (P < 0.05), whereas plasma glucagon, cortisol, and growth hormone, were higher in HT after 40 min. These results indicate that the hyperglycemia observed during exercise in the heat is caused by an increase in liver glucose output without any change in whole body glucose utilization.