The importance is emphasized of the inadequacy of the term 'body temperature', but that both core temperature and the peripheral temperature must be considered. The assessment of vasoconstriction is informative, and advantage should be taken of the exact information that can be provided by an electro-thermometer as opposed, for example, to a touch of the hand. The core temperature is taken from the rectum, the nasopharynx or tympanic membrane, and the peripheral temperature from the great toe. If the peripheral temperature falls the loss of body heat is prevented and the core temperature rises. If this is not understood or not recognized misguided attempts may be made to try to reduce the raised central temperature by cooling the skin. This can result in a further rise of central temperature. The recognition of peripheral vasoconstriction enables a proper assessment to be made of its basic cause and of the necessary treatment. The most important cause is hypovolaemia from blood loss and a detailed account is given of how this should be recognized, especially by a study of the gradient between central and peripheral temperatures. Such studies should be a routine in any severe illness, whether medical or surgical, in which circulatory deterioration is likely, especially after major operations or injuries. Study of the temperature of the heel (hindfoot) as well as of the toe (forefoot) can indicate a selective shutdown of peripheral circulation that reveals an especially critical circulatory state.