1. Twenty lightly anaesthetized dogs were cooled to 29°C by cold-water immersion. Ventilation was spontaneous and the animals were allowed to shiver freely. Metabolic heat production and respiratory heat exchange were measured during rewarming. 2. The animals were divided into four groups each of five dogs and each group was rewarmed by a different technique. The control group was allowed to rewarm spontaneously; a second group was given warm (45–50°C) fully humidified air to breathe in addition; a third group was rewarmed in a hot-water bath (42–44°C) and the remaining group was given a muscle relaxant to abolish shivering and rewarmed by warm inspired air only. 3. The group rewarmed in hot water achieved normal core temperature most rapidly but there was no difference in the rewarming rates of the group rewarmed spontaneously and of the group given warm air to breathe in addition. 4. The group given a muscle relaxant and rewarmed with warm inspired air required 12 h to achieve the same core temperature as the shivering groups achieved in 2 h. Compared with the heat produced by shivering the amount of heat which it was possible to transfer across the respiratory tract was so small that it did not materially influence the rate of rewarming.