The present study evaluates the efficacy of inhaling warm moist air as a method of rewarming from hypothermia in -20 degrees C field conditions. The method of inhalation rewarming is compared to two other methods of rewarming: a) passive rewarming; and b) passive rewarming, with a respiratory heat exchanger designed to minimize respiratory heat loss. Eight male subjects were rendered hypothermic by immersion in 15 degrees C water for 1 h. They were withdrawn from the tank earlier, in the event that their rectal temperature (Tre) decreased to 35 degrees C, or by 1.5 degrees C from the pre-immersion value. Upon completion of the immersion, they were placed in a well-insulated sleeping bag assembly and transferred to a cold room maintained at -20 degrees C for a 2 h rewarming period. They participated in 3 trials: Control-passive rewarming; Heat Treat-inhalation rewarming with the Heat Treat; HME-passive rewarming in conjunction with a respiratory heat and moisture exchanger (HME). During the rewarming period, inspired air temperature was -19.4 +/- 1.1 degrees C in the control trial. In the HME and Heat Treat trials subjects breathed via an oro-nasal mask. The inspired air temperature was +20.5 +/- 1.2 degrees C in the HME and +36.2 +/- 2.9 degrees C in the Heat Treat trial. The post-immersion drop in Tre was significant in all conditions. The reduction in the post-exposure drop in Tre observed with the Heat Treat may be attributed to the minimization of respiratory heat loss, since the magnitude of the reduction was similar to that observed with the HME.(ABSTRACT TRUNCATED AT 250 WORDS)