BACKGROUND: Recent studies have indicated that blood-brain barrier (BBB) disruption following su- barachnoid hemorrhage (SAH) significantly correlates with the development of brain injury and poor prognosis of patients subjected to SAH. OBJECTIVE: To investigate both functional and structural changes related to BBB in various phases after SAH in rats through quantitative and qualitative methods. DESIGN, TIME AND SETTING: This experiment, a completely randomized design and controlled ex- periment, was performed at the Department of Neurosurgery, the Second Affiliated Hospital of Chongqing University of Medical Sciences from June 2006 to March 2007. MATERIALS: A total of 128 female, healthy, Sprague-Dawley rats were selected for this study. Main reagents and instruments: Evans Blue dye (Sigma Company, USA), fluorescence spectrophotometer (Shimadzu Company, Japan), and transmission electron microscope (Olympus Company, Japan). METHODS: The included 128 rats were randomly divided into two groups: sham-operated group (n = 16) and SAH group (n = 112). Rats in the SAH group were divided into seven subgroups: 6, 12, 24, 36, 48, 60, and 72 hours after SAH (16 rats for each time point). Experimental SAH was induced by blood injection into the pre-chiasmatic cistern (300 μL). The sham-operated group received an equivalent volume of normal saline solution (300 μL) injected into the subarachnoid space. MAIN OUTCOME MEASURES: Brain tissue water content was determined by the wet-dry method. BBB permeability in the cerebral cortex was determined by Evans Blue dye and fluorescent spectrophotometer. The ultrastructural changes in BBB were observed with transmission electron microscope. RESULTS: Compared with the sham-operated group, SAH induced a significant increase in brain water content between 24 and 60 hours (F = 888.32, P < 0.05). Brain water content increased to a maximum by 36 hours after SAH, normalizing by 72 hours. Evans Blue content in the cerebral cortex of SAH group rats began to increase by 24 hours after SAH, peaked at 36 hours (F = 896.72, P < 0.05), and significantly de- creased at later observation times, finally normalizing by 72 hours after SAH (P > 0.05). Electron micros- copy demonstrated only a mild perivascular edema at 24 hours after SAH. By 36 hours, a notable perivascu- lar edema was associated with a collapse of the capillary. Astrocytic endfeet surrounding the capillary were prominently swollen in the edematous areas. The above-mentioned abnormal ultrastructural changes in the BBB were reversed by 72 hours after SAH. No obvious morphological changes in the BBB were detected in the sham-operated rats. CONCLUSION: These results directly suggest that SAH could induce rapid changes in BBB function and structure during the acute phases of BBB breakdown. Moreover, these dynamic alterations further indicate that the BBB possesses the ability to self-repair. BBB dysfunction plays an important role in the development of vasogenic edema and unfavorable outcome. BACKGROUND: Recent studies have indicated that blood-brain barrier (BBB) ​​disruption following su- barachnoid hemorrhage (SAH) significantly correlates with the development of brain injury and poor prognosis of patients subject to SAH. OBJECTIVE: To investigate both functional and structural changes related to BBB in various phases after SAH in rats through quantitative and qualitative methods. DESIGN, TIME AND SETTING: This experiment, a completely randomized design and controlled ex- periment, was performed at the Department of Neurosurgery, the Second Affiliated Hospital of Chongqing University of Medical Sciences from June 2006 to March 2007. MATERIALS: A total of 128 female, healthy, Sprague-Dawley rats were selected for this study. Main reagents and instruments: Evans Blue dye (Sigma Company, USA), fluorescence spectrophotometer (Shimadzu Company, Japan), and transmission electron microscope (Olympus Company, Japan). METHODS: The included 128 rats were randomly divided into two Rats in the SAH group were divided into seven subgroups: 6, 12, 24, 36, 48, 60, and 72 hours after SAH (16 rats for each time point). Experimental SAH was induced by blood injection into the pre-chiasmatic cistern (300 μL). The sham-operated group received an equivalent volume of normal saline solution (300 μL) injected into the subarachnoid space. MAIN OUTCOME MEASURES: Brain tissue water content was determined by the wet-dry method. BBB permeability in the cerebral cortex was determined by Evans Blue dye and fluorescent spectrophotometer. The ultrastructural changes in BBB were observed with transmission electron microscope. RESULTS: Compared with the sham- operated group SAH induced a significant increase in brain water content between 24 and 60 hours (F = 888.32, P <0.05). Brain water content increased to a maximum by 36 hours after SAH, normalizing by 72 hours. Evans Blue content in the cerebral cortex of SAH group r ats began to increase by 24 hours after SAH, peaked at 36 hours (F = 896.72, P <0.05), and significantly de- creased at later observation times, finally normalizing by 72 hours after SAH The 36 hours, a notable perivascu-lar edema was associated with a collapse of the capillary. Astrocytic endfeet surrounding the capillary were prominently swollen in the edematous areas. The above-condensation abnormal ultrastructural changes in the BBBs were reversed by the hours of SAH. No obvious morphological changes in the BBB were detected in the sham-operated rats. CONCLUSION: These results directly suggest that SAH could induce rapid changes in BBB function and structure during the acute phases of BBB dysfunctions that the BBB possesses the ability to self-repair. BBB dysfunction plays an important role in the development of vasogenic edema and unf avorable outcome