Objective:　To explore the variant processes of c ell apoptosis and the inhibiting effect of moderate hypothermia on cell apoptosi s after diffuse brain injury.rn　　Methods:　Models of diffuse brain injury were induced by the tra uma device reported by Marmarou.1 A total of 128 Wistar rats were divided into 4 groups: the uninjured group (Group A, n=8), the severely i njured group (Group B, n=60), the mildly injured group (Gr oup C, n=30) and the mild hypothermia group (Group D, n=30). In Group D, the severely injured rats were treated with moderate hypothermia to keep the rectal temperature at 32℃ (standard deviation for 0.1℃) for 6 hours. Then the morphosis, the characteristics and the qua ntity of apoptotic cells in the cerebral cortex and in the hippocampus regions a fter different severities of craniocerebral injuries were observed and compared under an electronic microscope, with terminal deoxynucleotidyl nick end labeling (TUNEL) in DNA fragmentation and with agarose gel electrophoresis.rn　　Results:　TUNEL showed apoptotic cells increased according to t he injury severity, and they peaked at 48 hours after injury and then declined. In Group C, apoptosis was located in the CA2 and CA3 areas of the hippocampu s. And in Group B, apoptosis increased evidently, and located in the whole hippo campus and in the frontal and parietal cortex regions. The hypothermia-treated rats had some apoptotic cells, too. However, even at 24, 48 and 72 hours after i njury there were significantly fewer apoptotic cells in the cortex and in the hi ppocampus in Group D than that in the non-treated groups. Electron microscopy s howed that the apoptotic cells were round and shrunken in morphology and the nuc lei were round and condensed at 24 and 48 hours after injury. And the apoptosis at 48 hours was more severe than that at 24 hours. The hypothermia-treated rats had no apoptotic cells. Gel electrophoresis showed that characteristic DNA “la dders” were observed in the cortex and in the hippocampus at 48 hours after sev ere injury. But there was no DNA “ladder” at other time points in the severely injured group, in the mildly injured group and in the hypothermia-treated grou p.rn　　Conclusions:　It suggests that apoptosis occurs after diffuse br ain injury and apoptotic cells increase with the injury severity. Moderate hypot hermia has a specific inhibiting effect on cell apoptosis after diffuse brain in jury in rats.