Introduction: Sporting activities are increasingly taking place in hazardous and hostile thermal environments (Brocherie et al., 2015). For example, the recent “World Marathon Challenge” requires to perform 7 marathons on 7 days on 7 continents in various terrain and climate. The first stage of the race is run in Antarctica (-20°C in February) and the second one is performed few hours later in South Africa (+35°C). Research on alternative exposure to opposite thermal stimuli to investigate the effect of parallel exposure to cold and heat is very limited (Tipton et al., 2008). Therefore, this case study aimed to investigate the effect of a short-term heat acclimation (STHA) on physiological responses and physical performance in successive cold (-20°C) and hot (+35°C) environments, likely simulating the two first stage of the competition. Methods: The subject was a 36 years old elite female ultra-endurance athlete with a background in polar expedition. During the experiment (January), she undertook approximately 8-10 h of outdoor running sessions per week, conferring a likely natural cold acclimatization. However, she had never experienced any acute exercise or acclimation/acclimatization in hot conditions (>35°C). Therefore, a STHA, consisting of 1.0-1.5 h of low-intensity aerobic exercise at 35-40°C per day for 6 days, took place during the taper phase of the training program in an environmental chamber. Before (Pre-) and after (Post-) the intervention, she completed a 110-min time-trial in the cold (-20°C) followed after 5 h 30 min of rest by a 110-min time-trial in the heat (+35°C). Performance (distance covered), peak and mean heart rate (HRpeak and HRmean), breathing rate (BRpeak and BRmean), rectal (Trec) and skin temperatures (Tskin) were continuously monitored. Results: From Pre- to Post-, HRpeak and HRmean decreased by -4.1% and -5.3% during exercise in the cold, while BRpeak/mean did not change. Peak and mean Trec dropped by -0.8% and -1.6% in the cold, respectively, after STHA. However, while mean Tskin decreased (-3.6%), Peak Tskin increased (8.5%). Time-trial performance in the cold improved by 12.2% after intervention. After STHA, performance in the heat also improved (+12.7%) and was accompanied by the generally reported physiological adaptation: decreases in HR (-3.1% and -2.7% for HRpeak and HRmean), BR (-24.6% and -23.3% for BRpeak and BRmean), Trec (-1.3% for both peak and mean values) and Tskin (-2.4% and -5.2%). Time to Trec of 39°C increased from 37.5 min to 102.5 min from Pre- to Post-. Conclusion: The universally accepted existence of heat acclimation/acclimatization was confirmed by the present findings. The novelty of this case study is that STHA (in addition to natural cold acclimatization) permits to improve performance in both cold and hot environments via hypothermic adaptation.