Moderate-intensity exercise sessions are incorporated into heat-acclimation and hypoxic-training protocols to improve performance in hot and hypoxic environments, respectively. Consequently, a training effect might contribute to aerobic performance gains, at least in less fit participants. To explore the interaction between fitness level and a training stimulus commonly applied during acclimation protocols, we recruited 10 young males of a higher (more fit-MF, peak aerobic power [VO_2peak ]: 57.9 [6.2] ml·kg^-1 ·min^-1 ) and 10 of a lower (less fit-LF, VO_2peak : 41.7 [5.0] ml·kg^-1 ·min^-1 ) fitness level. They underwent 10 daily exercise sessions (60 min@50% peak power output [W_peak ]) in thermoneutral conditions. The participants performed exercise testing on a cycle ergometer before and after the training period in normoxic (NOR), hypoxic (13.5% F_i O₂ ; HYP), and hot (35°C, 50% RH; HE) conditions in a randomized and counterbalanced order. Each test consisted of two stages; a steady-state exercise (30 min@40% NOR W_peak to evaluate thermoregulatory function) followed by incremental exercise to exhaustion. VO_2peak increased by 9.2 (8.5)% (p = .024) and 10.2 (15.4)% (p = .037) only in the LF group in NOR and HE, respectively. W_peak increases were correlated with baseline values in NOR (r = -.58, p = .010) and HYP (r = -.52, p = .018). MF individuals improved gross mechanical efficiency in HYP. Peak sweat rate increased in both groups in HE, whereas MF participants activated the forehead sweating response at lower rectal temperatures post-training. In conclusion, an increase in VO_2peak but not mechanical efficiency seems probable in LF males after a 10-day moderate-exercise training protocol.