-The blood-brain barrier (BBB) is a metabolic and physiological barrier important for maintaining brain homeostasis. The aim of this study was to determine the role of PKC activation in BBB paracellular permeability changes induced by hypoxia and posthypoxic reoxygenation using in vitro and in vivo BBB models. In rat brain microvessel endothelial cells (RMECs) exposed to hypoxia (1% O 2-99% N2; 24 h), a significant increase in total PKC activity was observed, and this was reduced by posthypoxic reoxygenation (95% room air-5% CO 2) for 2 h. The expression of PKC-II, PKC-␥, PKC-, PKC-, and PKC-also increased following hypoxia (1% O 2-99% N2; 24 h), and these protein levels remained elevated following posthypoxic reoxygenation (95% room air-5% CO 2; 2 h). Increases in the expression of PKC-⑀ and PKC-were also observed following posthypoxic reoxygenation (95% room air-5% CO 2; 2 h). Moreover, inhibition of PKC with chelerythrine chloride (10 M) attenuated the hypoxiainduced increases in [ 14 C]sucrose permeability. Similar to what was observed in RMECs, total PKC activity was also stimulated in cerebral microvessels isolated from rats exposed to hypoxia (6% O 2-94% N2; 1 h) and posthypoxic reoxygenation (room air; 10 min). In contrast, hypoxia (6% O2-94% N2; 1 h) and posthypoxic reoxygenation (room air; 10 min) significantly increased the expression levels of only PKC-␥ and PKC-in the in vivo hypoxia model. These data demonstrate that hypoxia-induced BBB paracellular permeability changes occur via a PKC-dependent mechanism, possibly by differentially regulating the protein expression of the 11 PKC isozymes. protein kinase C; paracellular; neurovascular unit; rat THE BLOOD-BRAIN BARRIER (BBB) is a metabolic and physiological barrier important for maintaining cerebral homeostasis. Brain microvessels that form the BBB are lined with specialized endothelial cells surrounded by pericytes, astroglial processes, and the extracellular matrix. Compared with the peripheral microvasculature, cerebral microvessels are highly specialized because they lack vesicular transport and fenestrations while having a high level of metabolic activity. This lack of fenestrations is due to the presence of tight junctions (TJ) and adherens junctions, which restrict paracellular movement of molecules across the BBB (29,31,40).Stroke is a leading cause of death and disability in the United States (3). It has been demonstrated that the BBB is compromised during stroke (29). The effects of stroke on the cerebral vasculature significantly contribute to the brain damage caused by stroke. It has been determined that the lack of oxygen (hypoxia, H) followed by reperfusion (posthypoxic reoxygenation, H/R) during stroke contributes to both neuronal and vascular damage. Both H and H/R cause increases in cerebrovascular permeability with concomitant increases in vasogenic cerebral edema (1,36,47).Previous studies (36, 47) have demonstrated that H and H/R cause changes in paracellular permeability to [ 14 C]sucrose in cerebral vascular endothelial cells....