Purpose: This study investigated the effects of heat exposure and previous experience on thermoregulatory and cardiovascular responses to performing a loaded march in the HEAT and on FORCE Combat (TM) circuit performance. Methods: Ten civilians (inexperienced) and 10 infantry reservists (experienced) performed a 60 min loaded march (similar to 35kg), in NORMAL (21 +/- 0.2 degrees C) and HEAT (30 +/- 0.2 degrees C) conditions and the FORCE Combat (TM) military physical performance evaluation. Participant groups were matched for morphology and physiological capacity. Results: Out of the 10 experienced participants that participated in the loaded march in HEAT, 9 completed the full 60 min but only 5 of 10 inexperienced participants were able to do the same. Performing a loaded march in the HEAT caused a state of uncompensable heat stress (continuous increase in core temperature) for both the inexperienced and experienced participants. Heart rate (134 +/- 12vs143 +/- 9bpm,p=0.027), rate of perceived exertion (13 +/- 1vs10 +/- 1,p=0.001) and thermal comfort (1.9 +/- 0.5vs2.4 +/- 0.4,p=0.011) were lower in the experienced compared to the inexperienced group during the loaded march in HEAT. The FORCE Combat (TM) completion times were higher in HEAT compared to NORMAL, but lower in experienced participants in both conditions (p=0.05). Conclusion: Both heat exposure and previous experience had an effect on cardiovascular, thermal and subjective measures during the loaded march and on completion time of the FORCE Combat (TM) circuit.