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Mental fatigue is a psychobiological state caused by prolonged periods of demanding cognitive activity and has been observed to decrease time trial (TT) endurance performance by ∼3.5% in normal ambient temperatures. Recently, it has been suggested that heat may augment the negative effect of mental fatigue on cognitive performance, raising the question whether it may also amplify the effect of mental fatigue on TT performance.In 30°C and 30% relative humidity, 10 endurance-trained male athletes (mean ± SD; age = 22 ± 3 yr, Wmax = 332 ± 41 W) completed two experimental conditions: intervention (I; 45-min Stroop task) and control (C; 45-min documentary). Pre- and postintervention/control, cognitive performance was followed up with a 5-min Flanker task. Thereafter, subjects cycled for 45 min at a fixed pace equal to 60% Wmax, immediately followed by a self-paced TT in which they had to produce a fixed amount of work (equal to cycling 15 min at 80% Wmax) as fast as possible.Self-reported mental fatigue was significantly higher after I compared with C (P < 0.05). Moreover EEG measures also indicated the occurrence of mental fatigue during the Stroop (P < 0.05). TT did not differ between conditions (I = 906 ± 30 s, C = 916 ± 29 s). Throughout exercise, physiological (HR, blood lactate, core, and skin temperature) and perceptual measures (perception of effort and thermal sensation) were not affected by mental fatigue.No negative effects of mild mental fatigue were observed on performance and the physiological and perceptual responses to endurance exercise in the heat. Most plausibly, mild mental fatigue does not reduce endurance performance when the brain is already stressed by a hot environment.