Objective To investigate the rule of the aquaporin 4 (AQP4) expression in acute ischemic brain edema, and to study the correlation between AQP4 expression and diffusion weighted imaging (DWI) Methods Thirty six Wistar rats were divided into 2 groups randomly, control group (n=12) and operation group (n=24) in which right middle cerebral artery of each animal had been occluded unilaterally (MCAO) at interval times of: 15 minutes, 30 minutes, 1 hours, 3 hours, 6 hours and 24 hours, respectively The operation process of the control group was the same as the operation group except for the MCAO All groups were examined using DWI The apparent diffusion coefficient (ADC), relative density (rd) and relative area (rs) of the biggest hyperintensity signal layer on DWI were measured After that the animals were sacrificed and perfused with the mixture solution consisting of TTC The biggest layers of the ischemic cerebral tissues in each rat corresponding to the DWI were stained with TTC and examined with immunochemistry (△S) , in situ hybridization (α) and histology Results There was no significant change in the control group In the operation group, a hyperintensity signal was found in the DWI of the right MAC territory at 15 minutes after MCAO The ADC value decreased quickly within one hour after MCAO, while the AQP4 expression, rd DWI and rs DWI increased rapidly during this stage As time progressed, the ADC value decreased further to (2 1±0 6)×10 4 mm 2/s at 3 hours, and then began to increase slowly till 24 hours But the AQP4 expression (△S and α) and rd as well as the rs continuously increased slowly between 1 hour and 6 hours after MCAO, followed a peak after 6 hours The AQP4 expression (α) showed a positive relationship with the rs DWI, they all presented two peaks and a plateau The corresponding sequential pathologic changes were a gradual increase of intracellular edema (within one hour), then an emergence of vasogenic edema (1-6 hours), and final necrosis and liquefaction (6-24 hours) Conclusions Upregulated expression of AQP4 may play a significant role in acute ischemic brain edema, especially during the stages of intracellular edema and necrosis, but it has no correlation to vasogenic edema Certainly, the high expression of AQP4 is perhaps one of the most important reasons of the decrease of ADC and hyperintensity on the DWI in the intracellular edema Objective To investigate the rule of the aquaporin 4 (AQP4) expression in acute ischemic brain edema, and to study the correlation between AQP4 expression and diffusion weighted imaging (DWI) Methods Thirty six Wistar rats were divided into 2 groups randomly, control group (n = 12) and operation group (n = 24) in which right middle cerebral artery of each animal had been occluded unilaterally (MCAO) at interval times of: 15 minutes, 30 minutes, 1 hours, 3 hours, 6 hours and 24 hours, respectively The operation process of the control group was the same as the operation group except for the MCAO All groups were examined using DWI The apparent diffusion coefficient (ADC), relative density (rd) and relative area (rs) of the biggest hyperintensity signal layer on DWI were measured after that the animals were sacrificed and perfused with the mixture solution consisting of TTC The biggest layers of the ischemic cerebral tissues in each rat corresponding to the DWI w ere stained with TTC and examined with immunochemistry (ΔS), in situ hybridization (α) and histology Results There was no significant change in the control group In the operation group, a hyperintensity signal was found in the DWI of the right MAC territory at 15 minutes after MCAO The ADC value decreased quickly within one hour after MCAO, while the AQP4 expression, rd DWI and rs DWI increased rapidly during this stage As time progressed, the ADC value decreased further to (2 1 ± 0 6) × 10 4 mm 2 / s at 3 hours, and then began to increase slowly till 24 hours But the AQP4 expression (△ S and α) and rd as well as the rs continuously increased slowly between 1 hour and 6 hours after MCAO, followed a peak after 6 hours The AQP4 expression (α) showed a positive relationship with the rs DWI, they all presented two peaks and a plateau The corresponding sequential pathologic changes were a gradual increase of intracellular edema (within one hour), then an em ergence oConclusions Upregulated expression of AQP4 may play a significant role in acute ischemic brain edema, especially during the stages of intracellular edema and necrosis, but it has has no correlation to vasogenic edema Certainly, the high expression of AQP4 is perhaps one of the most important reasons of the decrease of ADC and hyperintensity on the DWI in the intracellular edema