Environmental heat stress degrades aerobic performance; however, little research has focused on performance when the selected task elicits modest elevations in core body temperature (G38.5-C). Purpose: To determine the effect of environmental heat stress, with modest hyperthermia, on aerobic performance and pacing strategies. Methods: After a 30-min cycling preload at 50% V O 2peak , eight euhydrated men performed a 15-min time trial on a cycle ergometer in temperate (TEMP; 21-C, 50% RH) and

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... (HOT; 40-C, 25% RH) environments. Core and skin temperature (T c and T sk , respectively) and HR were continuously monitored. Performance was assessed by the total work (kJ) completed in 15 min. Pacing was quantified by comparing the percent difference in actual work performed in each of five 3-min blocks normalized to the mean work performed per 3-min block. Pace over the final 2 min was compared with the average pace from minutes 0 to 13 for end spurt analysis. Results: T c and HR rose continually throughout both time trials. Peak T c remained modestly elevated in both environments [mean (range): HOT = 38.20-C (37.97-38.42-C); TEMP = 38.11-C (38.07-38.24-C)], whereas T sk was higher in HOT (36.19 T 0.40-C vs 31.14 T 1.14-C), and final HR reached È95% of age-predicted maximum in both environments. Total work performed in HOT (147.7 T 23.9 kJ) was È17% less (P G 0.05) than TEMP (177.0 T 25.0 kJ). Pace was evenly maintained in TEMP, but in HOT, volunteers were unable to maintain initial pace, slowing progressively over time. A significant end spurt was produced in both environments. Conclusions: During a brief aerobic exercise time trial where excessive hyperthermia is avoided, total work is significantly reduced by heat stress because of a gradual slowing of pace over time. These findings demonstrate how aerobic exercise performance degrades in hot environments without marked hyperthermia.