Relationship between body and brain temperature in traumatically brain-injured rodents. Recent work has shown that mild to moderate levels of hypothermia may profoundly reduce the histological and biochemical sequelae of cerebral ischemic injury. In the present study, the authors examined the effect of fluid-percussion injury on brain temperature in anesthetized rats and the effect of anesthesia on brain temperature in uninjured rats. The relationship between the brain, rectal, and temporalis muscle temperatures during normothermia, hypothermia, and hyperthermia was studied following a moderate magnitude of fluid-percussion brain injury (2.10 to 2.25 atmospheres) in rats. The results showed that mean brain temperature in 10 anesthetized injured rats, in 21 anesthetized uninjured rats, and in 10 unanesthetized uninjured rats was a mean (+/- standard error of the mean) of 36.04 degrees +/- 0.20 degrees C, 36.30 degrees +/- 0.08 degrees C, and 37.95 degrees +/- 0.09 degrees C, respectively. There was no significant difference in temperature under general anesthesia between injured and uninjured rats (p greater than 0.05). In the absence of brain injury, mean brain temperature was significantly lower in anesthetized rats than in unanesthetized rats (p less than 0.001). In anesthetized brain-injured rats, temporalis muscle temperature correlated well with brain temperature over a 30 degrees to 40 degrees C range, even when brain temperature was rapidly changed during induction of hypothermia or hyperthermia (r = 0.9986, p less than 0.0001). In contrast, rectal temperature varied inconsistently from brain temperature. These observations indicated that: 1) brain injury itself does not influence brain temperature in this model; 2) anesthesia alone decreases brain temperature to levels producing cerebral protection in this model; and 3) external monitoring of temporalis muscle temperature can provide a reliable indirect measure of brain temperature in the course of experimental brain injury. The authors believe that it is essential to monitor or control brain temperature in studies of experimental brain injury. 