We hypothesized that hypoxia decreases energy intake and increases total energy requirement and, additionally, that decreased barometric pressure increases total water requirement. Energy and water balance was studied over 31 days in a hypobaric chamber at 452-253 Torr (corresponding to 4,500-8,848 m altitude), after 7 days acclimatization at 4,350 m. Subjects were eight men, age 27+/-4 years (mean+/-SD), body mass index 22.9+/-1.5 kg/m2. Food and water intake was measured with weighed dietary records, energy expenditure and water loss with labelled water. Insensible water loss was calculated as total water loss minus urinary and faecal water loss. Energy intake at normoxia was 13.6+/-1.8 MJ/d. Energy intake decreased from 10.4+/-2.1 to 8.3+/-1.9 MJ/d (P<0.001) and energy expenditure from 13.3+/-1.6 to 12.1+/-1.8 MJ/d (P<0.001) over the first and second 15-day intervals of progressive hypoxia. Absolute insensible water loss did not change (1.67+/-0.26 and 1.66+/-0.37 l/d), however, adjusted for energy expenditure it increased with ambient pressure reduction (P<0.05). In conclusion, hypoxia induced a negative energy balance, mainly by a reduction of energy intake. Overall insensible water loss was unchanged because the increase in respiratory evaporative water loss was counterbalanced by a decrease in metabolic rate that probably limited the hypoxia-induced increase in ventilation.