Objective To study the effects of traumatic brain injury with or without hypotension on the development and severity of brain edema using the impact- acceleration model.Methods Brain tissue water content (BWC) was measured by the gravimetric technique in the cortex and striatum following traumatic brain injury (TBI) . The hypotension was induced by the combination of α - chloralose and pavulon after TBI.Results The mortality of this improved model of TBI was 48.6 %, but the rate of skull fracture was 6.4 %. Within the first 24 hours after TBI, the BWC of cortex and striatum in the survival rats showed a slight decrease at first, and then an increase. There appeared no difference in BWC between the two groups of TBI and the control. Nevertheless, the BWC in the area of parietal cortex at 24 hours after impact injury slightly increased by 0.5 % in comparison with that at 8 hours after impact [ (79.1 ± 0.5) % vs. (78.6 ± 0.5) %, P ＜ 0.05]. Meanwhile,the BWC of striatum didn't show the difference. However, while TBI was associated with hypotension, the BWC of both parietal cortex [ (81.5±0.9)% vs. (78.6±0.5)%, P＜0.001] and striatum [ (78.5±0.9)% vs. (75.5±0.9)%, P＜0.001) were significantly increased, by comparison with survival rats at 8 hours after impact. Moreover, the amplitude of increase in BWC achieved about 2.9 %.Conclusion The rats, suffering TBI without hypotension, presented the slight brain edema at the relatively late stage; by contrast, the rats,suffering from TBI with hypotension, had severe brain edema at the early stage.