Lightly-clothed, nonexercising humans - 10 females and 11 males - were immersed in 0 degree C water for durations of 25-40 min until their core temperatures decreased to 35 degrees C. Ventilation rate increased 434% in the first 1-2 min of immersion, which increased the respiratory exchange ratio from 0.8 to 1.4. After 10 min of immersion, mean skin temperature had fallen to 5 degrees C and mean rectal and tympanic cooling rates were 6.02 and 5.40 degrees C/h, respectively. No sex differences occurred. By 15-20 min of immersion, maximum shivering metabolism was attained with levels nearly 4 times the preimmersion metabolic rate. This metabolic response was accompanied by heart rates in the range of 90-110 beats/min and increases in respiratory minute volume that were 250-300% greater than preimmersion. Predictions of survival time in 0 degree C water (based on hypothermia rather than drowning) were 1-1.5 h for the average person under the conditions of this study.