During the late 1970s, when oil exploration and extraction from the North Sea were at a peak, there was increasing concern about the number of episodes of unexplained confusion, loss of consciousness and deaths during dives. Previous field measurements on divers had demonstrated that divers became hypothermic with little or no sensation of cold, despite suit heating using tepid water pumped from the surface. This thesis describes laboratory experiments designed to document and to determine the cause of 'insidious' hypothermia. Initially, it was shown that uniform skin cooling in tepid water could produce subnormal body temperatures in all subjects tested, whether or not they had been acclimatised to cold. This symptomless fall in deep body temperature could be reversed by further chilling the hands and feet using a separate water circulation system, while the rest of the body remained in tepid water. The rise in deep body temperature was shown to be due to an increase in metabolic rate caused by shivering, with cold-acclimatised subjects shivering less. The main cause of 'insidious' hypothermia is therefore inadequate skin stimulation of thermoregulatory reflexes by lukewarm water, with previous cold water exposure further reducing responses. The next series of experiments was designed, to assess the impairment of memory and reasoning processes by oold, since most previous evidence find been inadequate or anecdotal. Psychological tests were administered during the unusual physiological circumstances on rewarming after oold immersion, where subjects felt warm and comfortable, but had a low or falling deep body temperature. The results clearly showed that the ability to form new memories was seriously impaired, even by mild falls in temperature, and that reasoning processes were greatly prolonged, although remaining accurate. The current work has therefore successfully determined the cause of the hypothermia which occurs in lukewarm water, and has shown that mental abilities are seriously affected early in the development of hypothermia.