These experiments examined the effects of moderate hypothermia on mortality and neurological deficits observed after experimental traumatic brain injury (TBI) in the rat. Brain temperature was measured continuously in all experiments by intraparenchymal probes. Brain cooling was induced by partial immersion (skin protected by a plastic barrier) in a water bath (0 degrees C) under general anesthesia (1.5% halothane/70% nitrous oxide/30% oxygen). In experiment I, we examined the effects of moderate hypothermia induced prior to injury on mortality following fluid percussion TBI. Rats were cooled to 36 degrees C (n = 16), 33 degrees C (n = 17), or 30 degrees C (n = 11) prior to injury and maintained at their target temperature for 1 h after injury. There was a significant (p less than 0.04) reduction in mortality by a brain temperature of 30 degrees C. The mortality rate at 36 degrees C was 37.5%, at 33 degrees C was 41%, and at 30 degrees C was 9.1%. In experiment II, we examined the effects of moderate hypothermia or hyperthermia initiated after TBI on long-term behavioral deficits. Rats were cooled to 36 degrees C (n = 10), 33 degrees C (n = 10), or 30 degrees C (n = 10) or warmed to 38 degrees C (n = 10) or 40 degrees C (n = 12) starting at 5 min after injury and maintained at their target temperatures for 1 h. Hypothermia-treated rats had significantly less beam-walking, beam-balance, and body weight loss deficits compared to normothermic (38 degrees C) rats. The greatest protection was observed in the 30 degrees C hypothermia group.(ABSTRACT TRUNCATED AT 250 WORDS)