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Objective: Hydration is an important logistical consideration for persons performing in austere environments because water demands must be balanced with the burden of carrying water. Methods: Seven novice climbers participated in a study to determine the hydration kinetics and core temperatures associated with a successful summit of Mount Rainier. Ingestible radio-equipped thermometer capsules were swallowed to monitor core temperature, and an oral dose of deuterium (0.12 ± 0.02 g·kg−1 body weight) was administered to determine hydration kinetics. Results: Mean core temperature throughout the 5.5-hour climb to Camp Muir (3000 m) was 37.6 ± 0.3°C. Water turnover was 95.0 ± 17.5 mL·kg−1 ·24 h−1 over the duration of the 43-hour study. There was a trend for reduced body mass from before (75.9 ± 13.0 kg) to after (74.8 ± 12.5 kg) the climb (P = .06), and urine specific gravity increased from before (1.013 ± 0.002) to after (1.022 ± 0.006) the climb (P = .004). Conclusions: Hydration demands of climbing Mount Rainier are highly elevated despite modest fluctuations in core temperature. Participants experienced hypohydration but were able to maintain sufficient hydration to successfully summit Mount Rainier and return home safely. [ABSTRACT FROM AUTHOR]