Abstract: Objective: To explore the pathological basis in the early stage of postdecompressive brain bulk enlargement. Metholds:Using the sensitivity and specificity of diffusion-weighted MRI(DWI) to detect brain edema,the dynamic DWI was performed in 18cats of postedecompressive brain bulk enlargement at three time points (2,4 and 6hours) after decompression. 18cats were rand omly assigned into three groups (2h group,4h group and 6h group).Every group was consisted of six cats killed at each corresponding time point after decompression,then made brain specimens to be observed with Evan's Blue staining,light and electronic microscope.Resutls:2hours after decompression,DWI shown the area of brain bulk enlargement was hypointensity signal.4 or 6hours after decompression,it expand ed and the midline of brain shifted more obviously to the contralater side.Only one cat,it returned into hyperintensity signal by 6hours after decompression.Evan's Blue staining area in swollen brain extravasated more obviously within 4 or 6 hours than within 2hours.Under light and electronic microscope,congestive dilation of micro and small cerebral vessels in double hemisphere was found within 2hours after decompression,but it was more obvious and accompanied with expand of the space around cerebral vessels in swollen brain.Degeneratio and Necrosis of cerebral cell multiplied in swollen brain by 6hours after decompression. Conclusion: In the early stage (<2hours) of postdecompressive brain bulk enlargement,the swollen brain was consisted of congestive dilation of cerebral vessels and vasogenic brain edema.The cytotoxic brain edema would be appeared at about 6hours following decompression.