Recent laboratory results have indicated that the ischemic brain is very sensitive to minor variations in temperature. This has created new interest in hypothermia and brain temperature. There is, however, very little information available regarding human intracerebral temperature and its relation to body core temperature during normal and pathological circumstances. We therefore made continuous measurements of the temperature of the lateral ventricle in 15 neurosurgical patients utilizing a newly developed technique with copper-constantan thermocouples introduced through a plastic catheter also used for monitoring intracranial pressure. The intraventricular temperature was higher than the rectal temperature during approximately 90% of all measurements. The largest temperature gradient measured was 2.3 degrees C. Usually the difference between the temperature of the rectum and the brain was much smaller, the mean value being 0.33 degrees C. For the patients in the most severe condition, the rectal temperature was sufficiently close to the brain temperature to afford a reliable basis for adequate clinical judgment.