Our objectives were to study the loss of heat from ischemic brain and to devise a method of maintaining brain temperature. Reversible forebrain ischemia was induced by carotid clamping and exsanguination in 30 anesthetized and artificially ventilated rats. Rectal, skull, and brain temperatures were measured, confirming previous findings that brain temperature falls by 4-5 degrees C during 15 minutes of ischemia unless measures are taken to maintain head temperature by external heating. Temperature gradients developed within the ischemic brain, superficial tissues being cooler than deep ones. These temperature gradients were reversed when skull temperature was maintained at core body (rectal) temperature by external heating. With rectal and skull temperatures maintained at 38 degrees, 37 degrees, 35 degrees, or 33 degrees C, brain temperatures nonetheless decreased by approximately 1 degree C during ischemia. This decrease in brain temperature could be prevented by placing the rat in a Plexiglas box with circulating air at temperatures close to that of the body core and a relative humidity of approximately 100%. We also found that, unless special precautions are taken, a temperature gradient develops between the brain and body core during recirculation.