The blood-brain barrier (BBB) influences brain levels of amyloid-β (Aβ) by transporting Aβ out of the brain (efflux) and by the reabsorption of cerebrospinal fluid (CSF) into the blood stream (bulk flow). In Alzheimer's disease (AD) and normal aging, unknown factors impair Aβ efflux and bulk flow in aging and in AD. These impairments have been proposed as mechanisms by which the Aβ burden in brain can increase. Impairment in Aβ efflux occurs in animal models of AD, including the aged SAMP8 mouse. Here, we show that CSF reabsorption is also reduced by about 50% in SAMP8 mice (p < 0.05). We then determined whether an antisense directed at the Aβ region of the amyloid-β protein precursor (AβPP) and previously shown to decrease brain levels of AβPP and to reverse the cognitive impairments of the SAMP8 mouse was able to reverse these impairments. We found that the antisense restored both the CSF reabsorption, more than doubling the rate of efflux, and the saturable efflux of Aβ. These findings suggest that AβPP/Aβ itself contributes to the impairments in bulk flow and saturable efflux of Aβ and that reduction of AβPP/Aβ levels can restore normal function of the BBB.