Changes in thermoregulation during spaceflight could result in an inability to tolerate ambient conditions or exercise tasks readily tolerated preflight. Weightlessness may alter heat production by changing metabolic rate, circadian rhythms of heat production, or work efficiency. It may impair heat loss by reducing convective and evaporative heat exchange. In addition, crewmembers may become less fit, less heat acclimated, hypohydrated, or have altered thermal sensitivity. Three scenarios are described: exercise conditioning in the mid deck, EVA, and emergency egress. Each scenario is discussed in terms of potential thermal challenges and possible consequences on crew performance.