Core body temperature is normally rigidly regulated by effective thermoregulatory responses that are triggered by small deviations in core and skin temperature. All general anesthetics so far tested markedly impair thermoregulatory control, increasing the range of temperatures not triggering protective responses by approximately 20-fold. Inhibition of thermoregulatory control--and reemergence of protective responses--are major factors influencing intraoperative temperature. Mild hypothermia provides dramatic protection against cerebral ischemia and therefore is frequently indicated during neurosurgery. Hypothermia to core temperatures near 34 degrees C can usually be instituted passively so long as thermoregulatory vasoconstriction is inhibited by sufficient anesthesia or neurosurgery per se. When core temperature must be rapidly reduced, or reduced to values approaching 32 degrees C, active cooling will usually be needed. Forced air appears to be the most effective clinically practical cooling method. Mild hypothermia is also associated with serious complications including myocardial ischemia, impaired resistance to surgical wound infections, coagulopathies, and postoperative shivering. Consequently, patients deliberately made hypothermic during neurosurgery should subsequently be actively rewarmed.