Cerebralcare Granule (CG) is a compound Chinese medicine used for treatment of headache and dizziness associated with cerebrovascular diseases. To clarify the mechanism underlying the clinical outcome of CG, this study investigated the effects of CG on the structure and function of cerebral microvasculature during I/R injury. A total of 138 Mongolian gerbils were included and divided into four groups, each composed of 36 or 30 animals, for evaluating various parameters of concern. A skull window was prepared for microcirculatory observation in animals, which were subjected to I/R with or without pretreatment with CG (0.4 or 0.8 g/kg). The velocity of red blood cells in the venules was observed by a high-speed video camera system, along with intravital confocal microscopic measurements of microvascular diameters, adherent leukocytes, and albumin leakage in the brain cortex. Changes in the fluorescence intensity of dihydrorhodamine 123 in cerebral microvessels and malondialdehyde level in the cortex were measured. The ultrastructure of the microvessels in the cerebral cortex was analyzed using both transmission and scanning electron microscopy. In addition, cerebral blood flow was monitored using the laser Doppler imaging technique. Pretreatment with CG (0.4 or 0.8 g/kg) significantly alleviated I/R injury-induced disorders in cerebral microvasculature, as evidenced by the data observed at 60 min of reperfusion wherein the values in CG (0.4 g/kg) pretreatment group, CG (0.8 g/kg) pretreatment group, and I/R group were 2.43 +/- 0.24, 2.28 +/- 0.18, and 6.00 +/- 0.35 for leukocyte adhesion, 2.51 +/- 0.40, 2.33 +/- 0.29, and 4.77 +/- 0.24 for albumin leakage, 7.06 +/- 0.81, 5.93 +/- 0.42, and 28.38 +/- 2.70 for dihydrorhodamine 123 fluorescence intensity in cerebral microvessels, 16.35 +/- 0.52, 14.34 +/- 0.68, and 21.46 +/- 0.71 for malondialdehyde level in the cortex, and 0.43 +/- 0.07, 0.46 +/- 0.02, and 0.17 +/- 0.08 for cerebral blood flow, respectively. I/R injury-elicited ultrastructural alterations in microvessels in cerebral cortex were also mitigated impressively by CG administration, manifested as attenuation of the reduced number of opening capillaries and the altered fine structures in endothelium, which were characterized by rough inner surface, increased intracellular vesicles, hypertrophy of digitations of intercellular contact, and swollen perivascular astroglial processes. Cerebralcare Granule is able to attenuate I/R injury-induced functional and structural changes in microvessels in the cerebral cortex of gerbils, an ability that is most likely correlated with its antioxidant potential.