The present investigation was designed to examine the influence of water temperature and prior hyperventilation on some of the potentially hazardous responses evoked by immersion in cold water. Eight naked subjects performed headout immersions of 2-min duration into stirred water at 5, 10, and 15 degrees C and at 10 degrees C after 1 min of voluntary hyperventilation. Analysis of the respiratory and cardiac data collected during consecutive 10-s periods showed that, at the 0.18-m/s rate of immersion employed, differences between the variables recorded on immersion in water at 5 and 10 degrees C were due to the duration of the responses evoked rather than their magnitude during the first 20 s. The exception to this was the tidal volume of subjects, which was higher on immersion in water at 15 degrees C than at 5 or 10 degrees C. The results suggested that the respiratory drive evoked during the first seconds of immersion was more closely reflected in the rate rather than the depth of breathing at this time. Hyperventilation before immersion in water at 10 degrees C did not attenuate the respiratory responses seen on immersion. It is concluded that, during the first critical seconds of immersion, the initial responses evoked by immersion in water at 10 degrees C can represent as great a threat as those in water at 5 degrees C; also, in water at 10 degrees C, the respiratory component of this threat is not influenced by the biochemical alterations associated with prior hyperventilation.