Considerable evidence indicates that brain temperature during ischemia affects the extent and distribution of ischemic injury. However, only limited data have been presented concerning the influence of temperature on ischemic damage after reversible focal cerebral ischemia. Because focal ischemic events of this type resemble conditions observed in the clinic, studies were undertaken to examine the effects of mild and moderate hypothermia on the extent of cerebral infarction after focal neocortical ischemia. Under halothane anesthesia, the left middle cerebral artery and both carotid arteries were occluded reversibly for a period of 3 hours in adult Sprague-Dawley rats. The animals were killed 3 days later. Brain sections were stained with triphenyltetrazolium chloride and analyzed for infarction using a computerized image analysis system. Temporal muscle temperature and rectal temperature were monitored continuously. The following groups with different intraischemic temporal muscle temperatures were analyzed: 1) control, 35.8 to 36.2 degrees C; 2) mild hypothermia, 33.0 to 33.5 degrees C; and 3) moderate hypothermia, 27.5 to 29.2 degrees C. The volumes of infarction were 214.5 +/- 17.9, 166.5 +/- 6.8, and 108.2 +/- 5.9 mm3 (mean +/- SEM) for the control, mild hypothermia, and moderate hypothermia groups, respectively. These findings demonstrate that both mild and moderate hypothermia reduce the impact of temporary focal ischemia in Sprague-Dawley rats.