Prions are the infectious agents associated with transmissible spongiform encephalopathies and are composed mainly of a misfolded form of the endogenous prion protein. Prion protein must enter the brain to produce disease. Previous work has emphasized various mechanisms which partially bypass the blood-brain barrier (BBB). Here, we used the brain perfusion method to directly assess the ability of mouse scrapie protein (PrPSC) to cross the mouse BBB independent of the influences of neural pathways or circulating immune cells. We found that PrPSC oligomers rapidly crossed the BBB without disrupting it with a unidirectional influx rate of about 4.4 mul/g-min. HPLC and capillary depletion confirmed that PrPSC crossed the entire width of the capillary wall to enter brain parenchyma. PrPSC also entered the cerebrospinal fluid (CSF) compartment. These results show that a prion protein can cross the intact BBB to enter both the parenchymal and CSF compartments of the brain. Published by Elsevier Inc.