PURPOSE. To examine the isolated and combined effects of severe hypoxia and a mild thermal challenge on performance, physiological measures, reaction time (RT) and serum brain derived neurotrophic factor (BDNF). METHODS. Nine trained male athletes (Age: 23±3y; Wmax: 332.6±45.0W; PL4) completed four experimental trials (CON: 15°C/0m, ALT: 15°C/3800m, TEMP: 25°C/0m, ALT+TEMP: 25°C/3800m) in a randomized, cross-over design. Subjects cycled for 30min in a self-paced test starting at 75%Wmax, their goal was to 'perform as much work as possible in 30min'. Power output (PO), heart rate, blood lactate, pulse oximetry and core and skin temperature were assessed. Before and after the time trial blood samples were taken for the analysis of BDNF and the psychomotor vigilance task was performed for the determination of RT and RT variability. Two-way (temperature x altitude) and three-way (temperature x altitude x time) ANOVA with repeated measures was used to examine main and interaction effects, paired t-tests were used to identify pair-wise differences. P<.05 was considered significant. RESULTS. Altitude had a detrimental effect on PO (main effect; P<0.001) and a significant interaction effect with time (alt x time; P<0.001). At sea level, in 15°C, subjects produced a mean PO of 243 ± 37W. At altitude, in 15°C, this mean PO was significantly decreased to 170 ± 46W (P<0.001). In 25°C, altitude decreased mean PO in a similar way as in 15°C (TEMP: 236 ± 41W; ALT+TEMP: 167 ± 47W; P<0.001). Altitude increased mean RT (P=0.022; 282±28ms at sea level; 289±32ms at altitude) and RT variability (P=0.020; 44±3ms at sea level: 50±4ms at altitude). Exercise increased BDNF (P=0.005; PRE: 21.8±8.4pg/mL; POST: 26.5±10.9pg/mL). The altitude- and temperature-induced changes in performance, physiological measures, RT and BDNF were independent from each other (P>0.05). CONCLUSION. Both stressors separately from each other significantly decreased exercise capacity, but we did not observe an interaction effect. Altitude induced an increase in RT and an increase in RT variability, presenting tentative evidence of mental fatigue after exercise in acute hypoxia. Exercise significantly increased BDNF, but no effect of altitude on the BDNF concentration was observed.* BR is funded by the FWO Fund for Scientific Research Flanders, Belgium.