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This study evaluated the benefits of a heat and moisture exchanger in the oro-nasal mask of a ventilating balloon for resuscitation in sub-zero ambients. Specifically, it tested the hypothesis that the reduction of respiratory heat loss would be similar to that achieved with mouth-to-mouth resuscitation. The prototype respiratory heat and moisture exchanger developed to minimise respiratory heat loss in subzero ambients was evaluated at ambient temperatures (Ta) of –20°C, –10°C, 0°C, 10°C and 20°C. It comprised a thermally insulated oro-nasal mask with a cylindrical heat and moisture exchanging unit protruding from the centre of the mask. The heat exchanging unit contains a »honeycomb« aluminium structure, providing a total surface area of 478.5cm² for the exchange of heat and moisture between the inhaled and exhaled air. A simulator comprising a hydraulically driven pump and a humidifier provided a ventilatory rate of 11.25 L/min. The simulated exhaled volume of air was maintained at approximately 37°C and saturated with water vapour. The HME was strapped to the head of a manikin, whose mouth was connected to the respiratory simulator. The entire arrangement was placed in a climatic chamber maintained at the desired temperature. At ambient temperatures ranging from –24°C to +22°C, the HME elevated the temperature of the inhaled air to within the range of +22°C to +29°C. The humidity of the inhaled air was maintained at 100% in all conditions. The results indicate that the HME effectively reduces respiratory heat loss in ambient conditions ranging from –20 to +20°C. An oro-nasal mask incorporating a heat and moisture exchanger would therefore be capable of significantly reducing respiratory heat loss during ventilation of hypothermic victims.