1. Ten male subjects were cooled on three occasions to a rectal temperature of 35°C by immersion to the neck in water at 11·3°C. The subjects were rewarmed for 60 min, once by metabolic heat production alone (shivering), once by inhalation rewarming with spontaneous breathing of saturated air at 47°C (control) and once by inhalation rewarming with ventilation regulated at 40 litres/min by respiring a controlled fraction of CO2 (hyperventilation).
2. Metabolic heat production was substantially reduced by inhalation rewarming (P < 0·05), from 913 kJ when shivering to 766 kJ (control) and 613 kJ when hyperventilating. The fall in metabolic heat production was greater than the corresponding respiratory heat gain, which increased from a loss of 41 kJ when shivering to gains of 85 kJ (control) and 169 kJ (hyperventilation).
3. As differences in mean skin temperatures were small (<1·0°C), it is concluded that the lower metabolic heat production in response to increased respiratory heat input must result from more rapid central temperature gains. This conclusion is supported by the relative values of rectal and tympanic temperatures. It was calculated that the percentage of the total heat supply which was donated to the core increased from 13% during shivering to 16% for the control and 23% in hyperventilation. Results imply that respiratory heat input is more efficient than metabolic heat production in elevating central temperature.