Recent studies in adult animals have shown that even small decreases in brain or core temperature ameliorate the damage resulting from hypoxic-ischemic insults. To determine the effect of minor reductions in ambient temperature either during or after an hypoxic-ischemic insult on the brain of the immature rat, 7-d-postnatal rat pups underwent unilateral common carotid artery ligation followed by exposure to hypoxia in 8% oxygen for 3 h. Control animals were maintained at 37 degrees C during hypoxia-ischemia. Intraischemic hypothermia was induced during the insult at temperatures of 34 degrees C and 31 degrees C. Postischemic hypothermia was induced by exposing rat pups that underwent hypoxia at 37 degrees C to recovering environments of 34 degrees C and 31 degrees C. Temperatures were recorded every 15 min from thermistor probes placed in the ipsilateral hemisphere and rectally. Neuropathologic alterations were assessed at 30 postnatal d. During hypoxia, animals became poikilothermic. Brain damage occurred in 90% of rat pups exposed to hypoxia-ischemia at 37 degrees C. Cerebral injury significantly decreased with decreasing temperatures during hypoxia-ischemia (p < 0.01). Only 30% of rats had brain damage when exposed to hypoxia-ischemia at 34 degrees C, and none of the rats exposed at 31 degrees C had brain damage. In contrast, there was no difference in the extent of cerebral injury between rat pups recovered under hypothermic conditions of either 34 degrees C or 31 degrees C compared with those recovered at 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)