BACKGROUND: While respiratory muscle fatigue is present following load carriage activity at sea level, the effect of environmental conditions on respiratory strength while undertaking load carriage is unknown. METHODS: The effect of thoracic load carriage during walks (5.5 km) in four environments [(thermo-neutral sea level (SL), 210°C (C), 4300 m (H) and 4300 m at 210°C (HC)] was evaluated on respiratory muscle fatigue. Ten subjects completed eight self-paced randomized treadmill walks comprising a variety of gradients, unloaded and loaded (18.2 kg), across the four environments. Respiratory muscle strength was measured via maximal inspiratory pressure (PImax) and expiratory pressure (PEmax) assessments. RESULTS: Submaximal walking in HC elicited respiratory muscle fatigue when compared to SL. Inspiratory muscle fatigue was independent of load. The relative change in PImax from baseline was significantly greater in HC compared to SL (9.6% vs. 1.3%). PEmax showed a significant reduction during HC (222.3 cmH2O, 214.4%) when compared to the other three environments. CONCLUSION: These results highlight the need to focus on respiratory muscle strength in preparation for exercise in cold hypoxic conditions.