The time course of blood-brain barrier (BBB) and blood-CSF barrier (BCSFB) responses to hyperosmolar mannitol infusion (HMI; 1.6 M) during chronic hypertension was investigated using <sup>14</sup>C-sucrose as a marker of barrier integrity. <sup>14</sup>C-sucrose entry into CSF of both spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats 2 min after HMI increased ∼7-fold compared to their respective control. The volume of distribution (V<sub>d</sub>) of <sup>14</sup>C-sucrose into brain cortex of SHR increased 13-fold 2 min after HMI while that in WKY rats increased only 4-fold. After HMI V<sub>d</sub> of <sup>14</sup>C-sucrose into the cortex of WKY, and CSF of both SHR and WKY remained steadily greater than their corresponding control for up to 30 min (p < 0.01), whereas in the cortex of SHR the V<sub>d</sub> of <sup>14</sup>C-sucrose reached control values 20 min after HMI (p > 0.05), indicating that after HMI the increase in paracellular diffusion of <sup>14</sup>C-sucrose into SHR cortex was not persistent, in contrast to WKY rats and CSF of both SHR and WKY rats. Electron microscopy of the brain cortex after HMI showed capillary endothelial cell shrinkage and perivascular swellings in the brain cortex, and in the choroid plexus opening of tight junctions were observed. Our results indicate disruption of both the BBB and the BCSFB after HMI in both SHR and WKY rats. The disruption remained persistent up to 25 min after HMI at the BBB of WKY rats and BCSFB in both animal groups, while in SHR the protective function of the BBB returned to control values 20 min after HMI.