It is confirmed that lower oesophageal temperature readings will give an approximate indication of the cerebral temperature in the absence of an open thorax or a rapid transfusion of cold blood. Nasopharyngeal readings are less reliable and should not be used on their own. Middle and upper oesophageal recordings are unsatisfactory because of ventilatory cooling. For an assured accuracy greater than ± 1.0°C, direct intracerebral readings are necessary. The brain at a depth of 4 cm is usually slightly warmer than the lower oesophagus or nasopharynx. More superficial readings may be affected by the ambient temperature and surgical procedures